Question

A tap can fill a cistern in 5 hours and another can empty it in 20 hours. If both the taps are opened simultaneously, how much time it will take to fill the tank?
A)6 Hours
B)16/3 hours
C)7 hours
D)20/3 hours​

Answer 1

Answer:

D) 20/3 hours

Step-by-step explanation:

Time taken by first tap to fill the cistern: 5 hrs

In 1 hour, fraction of cistern filled by first tap = 1/5

Time taken by second tap to empty the cistern = 20 hrs

In 1 hour, fraction of cistern emptied by second tap = 1/20

If both taps are opened simultaneously, every hour, the first tap will fill 1/5 of the cistern and the second tap will empty 1/20 of the cistern

Let's say time taken is t

When complete cistern is filled, fraction of cistern filled  = 1

1 = (1/5 - 1/20) * t                    [- 1/20 because it is emptying the tank]

1 = 3/20*t

t = 20/3 hrs.

Answer is hence, D) 20/3 hours

Hope it helps!

Forgive any silly mistakes :P

Answer 2

C)16/3hours the cistern can filled