Question

A cistern has two inlets A and B which can fill it in 21 minutes and28 minutes respectively. An outlet C can empty the full cistern in 14 minutes. If all the three pipes are opened together in the empty tank. In how much time will they fill the cistern?

Answer 1

Answer:

Step-by-step explanation:

A can fill the cistern in 21 mins.

Thus it fills 1/21 of the cistern in 1 min.

B can fill the cistern in 28 mins.

Thus it fills 1/28 of the cistern in 1 min.

C can empty the cistern in 14 mins.

It can empty 1/14 of the cistern in 1 min

Thus, in 1 min

1/21+1/28-1/14 cistern is filled

1/21-1/28 cistern is filled

1/7(1/3-1/4) cistern is filled

1/7*1/12 cistern is filled

1/84 cistern is filled

Therefore, the cistern is filled in 1/1/84 mins=84 mins or 1 hr 24 mins