# [Youtube] Aptitude PnC – Number of Handshakes and Invitations in a party, and Solved APFC questions

These questions keep reappearing in Aptitude segment of any competitive exam. XYZ number of people were invited in a party, how many handshakes were done etc. etc. These two videos should clear the concept.

At the end of a party, every person shook hands with other and total 28 handshakes were exchanged. Find out how many people attended this party? (APFC 2004)

# Concept #2: Inviting one or more people to a Party

Amit has 7 friends, whom he wishes to invite to a party. Out of his 7 friends, 1 or more may accept the invitation.
In How many ways can Amit’s 7 friends attend the party?

# More videos

### अब तक 14 कमेंट्स लिखी गयी

1. Surendra says:

2. Renukha says:

Hi Mrunal.Can you please teach us in English?

thanks sir,i really got my concept cleared

4. hitesh agrawal says:

amazing sir …..

5. happy says:

great effort sir……..

6. abhinandan says:

Dear Mrunal.. Check the solution of second problem Concept 2.
When you subtract 1 from 2^n, you exclude the case when “all the 7 friends decline the invitation”.
Answer will be 2^n. it will include all the cases including “all the friend decline or “accept the invitation”.

• Mrunal says:

….. you exclude the case when “all the 7 friends decline the invitation”.

please watch the video again, to understand why we deducted 1 from 2^n..

how many ways choose 1 or more out of n? =2^n-1
how many ways choose 0 or more out of n?=2^n

the question itself mentions “…one or more friends may accept the invitation”
so we’ve to remove the case where nobody accepts the invitation (7C0=1)
that’s why it is deducted from 2^n. because 2^7= sum of all cases including 7C0

7. ashasriram says:

thank you sir.happy independence day

8. shanthappa says:

Hi Mrunal sir, Is n(n-2)/2 formula applies only for handshake kind of problems or others problems also..plz make clear sir! Thanks in anticipation

9. divya says:

mrunal sir,
very well explain..u r our mentor.many many thanks!!

10. alam says:

very nice ……have you syllogism video at you tube

11. alam says: