# Case: Dying villagers and water supply

Ramgarh had population of 120 people and water supply for 210 days.

But after 10 days, 40 villages die.

How long will the remaining water last?

We can solve it, using our Universal STD table method^{TM}.

Suppose 1 village needs “L” litres of water per day.

Therefore 120 villagers need 120xL litres of water per day. <–This is our “speed”

120 villagers | |

Speed (Daily need) | 120L |

Time (water can last) | 210 days |

Distance (amt of water) |

Run STD on first column.

Speed x time = distance=(120 x“L”) x 210

**Given**: 40 villagers died after 10 days.

It means for the first 10 days, there were 120 villagers, right ?

Update the table

120 villagers | 10 days | |

Speed (Daily need) | 120L | 120L |

Time (water can last) | 210 days | 10 days |

Distance (amt of water) | 120L x210 | ?? |

Run STD on last column to find out the water consumed by 120 villagers in 10 days

Speed x time = distance

120L x 10 = distance.

## Remaining water after 10 days

=total water **minus** amount of water used in 10 days

=[120L x 210] MINUS [120L x 10]
=120L [210-10]
=120L (200)

So, we’ve this much water left and there are 120 minus 40 = 80 villagers surviving.

They consume 80xL = 80L litres of water per day.

Make a new column and fill up the table.

120 villagers | 10 days | Remaining | |

Speed (Daily need) | 120L | 120L | 80L |

Time (water can last) | 210 days | 10 days | ?? |

Distance (amt of water) | 120L x210 | 120L x 10 | =120L (210-10) =120L (200) |

## Run STD on last column

Speed x time =distance

80L x time =120L x 200

Time=(120L x 200) / 80L

Time=300 days.

**Final answer**: the water will last for 300 days.

# Case: Constant villagers and Water supply

This one was actually asked in UPSC CSAT 2011.

A village having a population of 4000 requires 150 litres of water per head per day. It has a tank measuring 20 m x 15 m x 6 m. The water of this tank will last for how many days?

Tank capcity given in meters but water consumption given in litres, Otherwise, There is no “dum” in this sum.

But Units must be same.

1 cubic meter=1000 litres.

Fill up the STD table

4000 villagers | |

Speed (Daily need) | 4000x 150L |

Time (water can last) | ?? |

Distance (amt of water) | 20x15x6 m^{3}=20x15x6x1000 lit. |

Run STD

Speed x time = distance

(4000 x 150) x time = 20 x 15 x 6 x1000 m^{3}

Time =(20 x 15 x 6 x 1000) / (4000 x 150) =3 days.

**Final answer**: water will last for 3 days

# Mock questions

- Rates of Wedding reception party: 8 food-items for 50 guests = total bill is Rs.1800. If Jethalal has total budget of Rs.21060 and planning to invite 90 guests for his son’s wedding, how many “food-items” can he have in the reception?
*(Ans.52 items)* - One bale of grass costs Rs.150. Lalu has 25 buffalos. They eat up 5 bales of grass in 12 days. If Lalu had only 10 buffalos, how much money will he need, to feed them for 18 days?
*(Ans.Rs.450)*