Question

Two taps can fill a cistern in 15 hours
taps can fill a cistern in 15 hours and 20 hours respectively. A third tap can empty it in 30 hours
How long will they take to fill the cistern if all the taps are opened​

Answer 1

Answer:

12 hrs.

Step-by-step explanation:

Cistern filled by Tap 1 will in 1hr = 1/15.

Cistern filled by Tap 2 will in 1hr = 1/20.

Cistern emptied by Tap 3 will in 1hr = 1/30.

The three taps when opened simultaneously will fill cistern in 1hr

= 1/15 + 1/20 - 1/30 = 20+15 - 10/300 = 25/300 = 1/12.

Thus the cistern will be filled in 12 hrs.