Question

Two pipes a and b can fill a cistern in 10 and 15 minutes respectively. Both fill pipes are opened together, but at the end of 3 minutes, b is turned off. How much time will the cistern take to fill ?

Answer 1

P can fill cistern in 12 minutes.

P fills cistern in 1 minute = 1/12

Q can fill cistern in 15 minutes.

Q fills cistern in 1 minute = 1/15 part.

P and Q together can fill cistern in 1 minute,

= {(1/12) + (1/15)} = 9/60 part.

So, they can together fill cistern in 3 minute,

= 3 * (9/60) = 9/20 part.

Rest Cistern = 1 - (9/20) = 11/20 part.

11/20 part cistern must be filled by Q in,

[(11/20)/(1/15)] = 8(1/4) minutes.

Hope this helps you