- Situation: Entry of Element-Increase in Avg
- Situation: Entry of 1 element – Decrease in Avg
- Situation: Removal of 1 element -increase in avg
- Situation: Removal of 1 element -decrease in avg
- Mock questions
Situation: Entry of Element-Increase in Avg
Let’s start with really simple case. In Gokuldham society, the average age of four kids is 14 years. If Mr. Champak is included in this group, the average age increases by 11 years. How old is Mr.Champak?
Convert this into money problem
- Four kids got Rs.14 each.
- Champak makes a new gang with rule, “everyone must have Rs.25 (bcoz 14+11=25)”
- But those kids got only Rs.14. so how much cash should Champak bring, to form this new gang?
Let’s think about it
- Those kids got only Rs.14 each and want to join the new group.
- But each of them would need Rs.11 extra.
- So Champak must give them Rs. 11 x 4 kid = Rs.44
- And Champak also wants to be a part of this gang, so he should also bring Rs.25 for himself.
- Therefore, Champak must bring Rs. 44 (to give to those kids) + 25 (for himself) =44+25=69
- Hence final answer: age of Champak is 69 years.
Observe how numbers are moving while the average changes.
Case: Avg age of Students + Teacher
Average age of 36 students is 14 years. If teacher’s age is included, then average increases by one. Find the age of teacher. (RBI)
We can solve it by two approaches
- Convert into Money problem (easy, quick)
- Formula method. (lengthy, time consuming)
Approach #1: convert into money problem
Whenever you face such question on average, try to convert it into a money problem, then it becomes very easy to visualize.
Converting into money problem
- 36 kids had Rs.14 each
- 1 teacher comes.
- New average is 15 (original 14+ increases by 1)
- How much cash does teacher have?
Ok let’s think about it.
- There is a gang of 36 kids. And it has rule: everyone must have Rs.14.
- Teacher wants to form a new gang. And it has rule: everyone must have Rs.15.
- But those kids have only Rs.14 each.
- So teacher must give Rs.1 to each of those 36 kids.
- Meaning teacher must bring Rs.1 x 36 =Rs.36
- And teacher should keep Rs.15 aside for himself. (because new gang rule is “everyone must have Rs.15)
- So how much cash did teacher bring in? =Rs.36 (to give to kids) PLUS Rs.15 (for himself)
- =36+15=Rs.51 that’s our final answer. Age of Teacher is 51 years.
Approach #2: Formula method
Formula for average: average = sum of observation divided by number of observation
Situation 1: without teacher
36=(sum of kids)/14
Therefore, sum of kids(s)=36 x 14
S=504
Situation 2: with teacher
37 =((sum of kids)+age of teacher))/15
Therefore, S + teacher=37 x 15
Now plugin the value of s=504 from previous equation
504+teacher =555
Teacher =555-504=51 that’s our final answer.
As you can see, the problem with this “formula” method = you’ve to do lengthy multiplications (37 x 15 and 36 x 14). Thus
- it breaks your momentum, decreases your speed in the exam.
- you might make some silly mistake in multiplications or during “plugging” the values in equations.
So better just try to solve such questions using “money problem” method.
Case: Avg salary workers+manager
The average salary of 20 workers is Rs.1500. If manager’s salary is added, this average increases by Rs.100. Find the salary of Manager.(RRB)
It is already in “money problem” format.
- 20 guys have Rs.1500 each
- Now Manager wants to increase this average by Rs.100. (new avg =1500+100=1600)
- So manager must give Rs. 100 to each of these 20 guys= manager must bring total 100 x 20 =Rs.2000 with him.
- But this new group has a rule “everybody must have Rs.1600”. Hence, If manager wants to be part of this “new” group, he must keep Rs.1600 aside for himself.
- So, how much cash should manager bring? =Rs.2000 (to give to workers) + Rs.1600 for himself
- =2000+1600
- =3600 this is the final answer.
Case: Avg weight students+teachers
There are 24 kids in a class with average weight of 35 kilograms. If Teacher’s weight included, the average increases by 400 grams. Find weight of teacher.
Convert this into money problem
- 24 kids have 35 rupees each
- Teacher wants to form new gang: “everyone must have 35.4 rupees each” (because 400 gms =400/1000 kg =0.4 kg)
Ok let’s think about it
- Teacher must give 0.4 rupees to each of those 24 kids=24 x 0.4 =9.6
- And he must bring Rs.35.4 for himself.
- so how much cash should teacher bring in? =9.6 (for kids) + 35.4 (for himself)
- =9.6+35.4
- =45. That’s our final answer: weight of teacher is 45 kilos.
Case: original average not given
In his 11th inning, a batsman scored 216 runs and his average increased by 12 runs. What’s his new average? (Bank)
Money problem
- 10 guys had Rs.”m’ each
- New guy (11th) has Rs.216 and he forms a new gang.
- In this new gang of 11 guys, each person has Rs.(m+12) each. Find value of m+12.
Let’s think about it:
- New guy wanted to form a new gang where everyone had Rs.m+12.
- But those 10 guys from old gang, had only Rs.m
- That means new guy should give each of those 10 people Rs.12
- So new guy gave away Rs.12 x 10 =Rs.120
- So new guy is left with Rs. 216 (he originally had) MINUS Rs.120 (that he gave away)
- 216-120=Rs.96 that’s our answer.
so far we saw “Average: Entry of 1 Element=>leading to Increase in Average”.
Now let’s check “Average: Entry of 1 Element => leading to decrease in average”
Situation: Entry of 1 element – Decrease in Avg
Case: younger Kid joins: average goes down
The average age of six kids is 12 years. A new kid joins the gang and average decreased by one year. What is the age of this new kid?
Money problem
- Six kids got 12 rupees each.
- New kid wants to form a new gang with rule “eachone must have only Rs.11”
Ok let’s think about it
- Since average is declining, we are sure that new kid has less money than Rs.11.
- But he wants everyone to have (12-1)=11 rupees.
- He tells those 6 kids, “each of you give me 1 rupees.” = 6 x 1 = Rs.6 borrowed.
- So how much money did that new kid bring? Well, Now he is part of new gang so obviously he has Rs.11 .
- but he has borrowed Rs.6 from others.
- Meaning, he originally had 11-6 =Rs.5
- Final answer: age of new kid is 5 years.
Let’s try a case where one element is removed.
Case: Average Runs decline (original not given)
A cricketer made 90 runs in his 12th inning, and his average declined by 5 runs. Find new average.
Money problem
- 11 guys had average Rs.m each
- 12th guy has Rs.90 and he makes new gang, with rule “everyone must have Rs.(m-5)”
Let’s think about it
- Since average is declining, we can be sure that new guy has less money than previous average (“m”).
- But he wants everyone to have (m-5) rupees (he should also have m minus 5 rupees.)
- So he tells those 11 people, give 5 rupees each to me. = 11 x 5 = Rs.55
- How much cash does the new guy have now?= 90 (he originally had) + Rs.55 (that he borrowed)=Rs.90+55=145 that’s our new average.
- Final answer: new average runs after 12 innings is 145.
Situation: Removal of 1 element -increase in avg
A batsman played 30 innings with an average of 78 runs.
If we ignore the score of his last innings, then average increases by 2 runs. How many runs did he score in last inning?
Money problem
- 30 guys with Rs.78 each
- 1 guy (Jethalal) leaves this gang after settling his debts.
- Average of remaining 29 guys is (78+2)=80 runs.
Ok let’s proceed
- Since average is increases after member leaves the gang= this member (Jetha) had less money than others.
- That means other people had lended him some money (to raise him to the average of 78.)
- When Jethalal leaves the gang, remaining guys get 2 rupees each.
- That means Jetha gave them Rs. 2 to each of those 29 remaining guys.=2 x 29= Rs.58
- And once upon a time, Jethalal was part of this gang, so he had Rs.78.
- It means Jetha’s original money = Rs.78 minus Rs.58 (that he borrowed)=78-58=Rs.20
- Final answer: batsman’s score in last inning was 20 runs.
Situation: Removal of 1 element -decrease in avg
Four members of a family earn average Rs.7350. One of them, leaves and average goes down to Rs.6500. How much did that person earn, who has now left the family? (SSC)
Money problem in our format:
- 4 guys with Rs.7350 each
- One of them (known as Jethalal) steals money from others and leaves.
- Now 3 guys left and they’ve Rs.6500 each
Ok let’s think about it
- Jetha wants to bring down average from 7350 to 6500 =decline of 850.
- So he must steal Rs.850 from each of the remaining three guys =Rs.850 x 3=2550.
- And since Jetha was once part of this “gang”, he too used to earn Rs.7350 himself.
- Therefore total cash with Jethalal =7350 (he originally had) + 2550 (that he stole) =Rs.9900.
Final answer: the income of person who left family, is Rs.9900
Mock questions
- Average age of 30 students is 11 .When teacher’s age is added, average increases by 1 year. Find the age of teacher.
- Average of 24 wrestlers is 90. A new wrestler joins the gym and average increases by 2 kilos. Find weight of new wrestler.
- Average annual income of 25 families in a society is 1.9 lakhs. A new family comes to live in this society and the new average annual income of the whole society is increased by 10,000 rupees. Find the monthly income of this new family.
- Average salary of 21 workers+1 manager is Rs.2000. When manager’s income is not considered, the average declines by 25%. How much does Manager earn?
- A batsman makes average 50 runs in 36 matches. After 37th match, his average increased by 1 run. How many runs did he score in last match?
- A batsman made 102 runs in 21st inning and his average increased by 2 runs. Find total runs scored by him in first 20 matches.
- A batsman scored 199 runs in his last match and his average increased from 44 to 49. Find total number of matches played by him.
- A thief steals average 2 cars per night for 13 nights continuously. But after 14th night’s “adventure”, his average increases to 3 cars. How many cars did he steal in that last night?
- Average salary of 21 workers is 2000. A new worker joins the company and overall average declines by 2.5%. What is the salary of new worker?
- A batsman made average 80 runs in 29 matches. After 30th match, his average declined by 2 runs. How many runs did he score in that last match?
- In a dance troupe, average weight of 20 junior dancers is 60 kilos. If the weight of lead dancer is also considered, the average declines by 500 gms. How much does the lead dancer weigh?
- In a society, average annual income of 30 families is 9 lakhs. One new family starts to live here and average declines to 8.8 lakhs. Find the annual income of this new family.
Answers
- 42
- 140
- 4.5 lakhs annually / 12 = 37500 monthly income.
- Decline of Rs.500. So manager had Rs.500 x 21 + 2000= Rs.12500
- 87 runs.
- 20 x 60 = 1200 runs.
- 30 matches. he kept 49 for himself and gave 5 rupees each to “m” number of people. 199=49+5m. so m=30 matches.
- 16 cars. (Rs.3 for himself + 1 x 13 donated=16 cars stolen last night)
- 2.5% decline= 50 rupees on 2000. so 21 workers gave him 21 x 50 =1050. and he is part of new gang= he has 1950 minus 1050 borrowed =His salary is 900.
- 20 runs.
- 49.5 kilos.
- 2.8 lakh.
![[Reasoning] Calendar Questions: Finding day or date, concepts, shortcuts explained](https://mrunal.org/wp-content/uploads/2013/01/i-lr-500x383.png)
thank u sir,
it’s good article..
thanku thankuu thankuu:) time saving and interesting as well!!!
Mrunal sir, you are really helping to crack the csat. thanks alot sir god always bless you sir.
Respected sir,
It is difficult to understand the new gas price control formula given by rangrajan committee….n why it is widely discussed?
Can you explain this ?
Thnx sir!
My Answer for Question No. 7 is= 31 Match.
If it is 30 match than (29*44+199)/30 = 49.167
And if 31 match than (30*44+199)/31 = 49
So Answer should be 31.
God Bless You Sir Ji
Dear Sir,
Its really helpful for me to preopare for SSC exam. Thanks a lot
mrunal sir please post some article on logical reasoning and decision making ………….thanks sir for ur wonderful job.
the yeomanry of murnal sir is highly commendable.
sir please upload methods to solve tricky algebra questions based on algebraic identities.
Help i can’t solve 6th question,i keep getting 1240 as the answer.
Mrunal bhai is the Linux of ias coaching… Free, extremely useful and very interesting Jai ho :-)
Ques- a batsman’s runs just before last match of the season, add up to 750. In his last two innings, he scores only 6 runs. And his average drops by 2. Find his final average of the season.
Ans-28. How?
Thanx
Solution:
Let,
total number of matches played to score 750 runs = x——-(1)
let its average be y——(2)
from 1 and 2 we get eq’n as 750/x= y i.e average——-(3)
given from the question:
runs scored in last 2 matches = 6
overall average drops by 2
(750+6)/(x+2) = y-2
using eqn (3) and putting value of y= 750/x,we get
756/(x+2) = 750/x – 2
756/(x+2) = (750-2x)/x
upon cross multiplication
756x = (x+2)(750-2x)——(4), a quadratic equation
solving (4), get 2 values of x ,neglecting the -ve root, we get x = 25
put value of x in eq’n (3)
750/x=y
750/25= y
so, y = 30. i.e average of all matches but last 2 = 30
A.T.Q last 2 matches post 750 runs brought down average by 2
hence final average of the season = 30-2
= 28 (answer)
In his 11th inning, a batsman scored 216 runs and avg, increased by 12 runs. what is the new avg ?
If x is avg. upto 10th inning, then
11(x+12) = 10x + 216 With this i m getting 84 and sir’s ans is 96. Plz……help me where i m wrong with this formula ?
imagine there were 10 kids and each had “x” rupees. so that was the old average.
11th kid comes with 216 rupees and raises average to x+12
meaning, he gave all those ten kids, 12 rupees each. then everyone will have same amount of money. (i.e. the new average)
so how much is left with 11th kid? 216-10×12=216-120=Rs.96= that’s our new average.
HI
Mrunal sir! you truly rock :)
Can anyone explain me Question no. 6
Thanx
Let the avg of 20 innings be A.
=> avg of 21 innings = A + 2
total runs scored in 21 innings = 21 * (A+2) ……… eqn 1
total runs scored in 20 innings = A*20 ……… eqn 2
and runs scored in 21st inning = 102 ……… eqn 3
total runs scored in 21 innings (add above 2 eqns) = (A*20) + 102 ……… eqn 4
both eqn 1 & 4 are actually the same thing and hence equate them
21(A+2) = (A*20) + 102
A=60
Now total runs scored in 20 innings = A*20 ……… eqn 2
= 1200
Cross chk new avg = 62
total runs = 62*21 = 1302
runs socred in 20 innings = 1200
therfore runs scoredin last inng = 1302-1200 = 102 which is given in the question.
Thank You Manu Jha for the help !!
:)
Q6:Let the average of 20matches=r
Runs scored in 20 matches=20r
Runs scored in 21st match=102
total score in 21 matches=20r+102
average of 21 matches=20r+102/21=r+2 (av.increased by 2)
20r+102=21r+42
r=60
hence runs scored in 1st 20 matches
=20r=20*60=1200
Q: the av.marks obtained by 120 students is 35. If the av.of passed candidates is 39 and that of failed cand.is 15,find the no.of cand.who passed the exam.
100
LEITVs QUESTION
Sol: Total no.of students=120
let passed =y
failed =x
.-.x+y=120 –(1)
av.of 120 stds=35
.-.tot.marks obtained by 120 stds=120*35=4200
marks of passed=39y
marks of failed=15x
.-.15x+39y=4200-(ii)
subtract i from ii
we get
(x+y=120)*15
15x+15y=1800-i
15x+39y=4200-ii
——————–
24y=2400
y =2400/24=100 ans
by elimination method
no.of passed students=100
let the no of passed students be x
obviously, the no of failed students will be (120-x)
so, total marks obtained=marks obtained by passed students+ marks obtained by failed students
x*39(passed students marks)+(120-x)15(failed student marks)=total marks i.e,120*25=4200
Solve and u will get 100(i.e,x no of passed students).
Thanks Rakesh.
Hats off to u mrunal sir…I think u should also discuss at mensuration topics( Sphere, cone and cylinder)and some important topics of triangle(geometry). i think u r the best explainer. u easly explain each and every topics.
Truly Awsom!!! Hats off 2 u.. :)
hii Mrunal Sir,
Could u pls make me understand Ques No 6, With your Money distribution funda
I have a question, but not able to solve by ur method, can u pls solve and explain.
The average of 50 numbers is 30. If two numbers, 35 and 40 are discarded, then the average of the remaining numbers is nearly….
a) 28.32
b) 28.78
c) 29.27
d) 29.68
average given 30 of 50 nos
total sum 50*30=1500
2 nos discarded
remaining num 48
their sum 1500-75=1425
now 1425/48= 29.68
50 numbers av= 30, the 2 nos are > average so our answer will be < 30
excess than average of 2 numbers = 35+40-60 = 15
this 15 will be used to increase average of other 48 numbers to 30, so increase in 1 number = 15/48 = .3125
previous average = 30-.3125 = 29.6875 so ans is D
Mrunal Sir…..U R GREATTTTTTTTTTTTTTTTTTT….GOD BLESS YOU.
are you from kolkata?
Yes i m frm kolkata
Mrunal sir, you are really amazing, thank you !
I was wondering if the answer of Q 7 should be 31 (including the match that increased the avg) ?
Yes
sir, thanq very much….
Dear
Very good questions are discussed and the short method is also very good yhis nice website for the people who are going to learn short cuts
Absolutely no doubt. I fully comrehended this one. Many! Many! thanks,MRUNAL BHAIYA/SIR
My vote=*****
Mrunal sir.. I think u r d only reason for so high cut offs of ssc cgl..
u explain each topic so well…
u r so good… dil se thanx to yu sir… :) :)