Two pipes P, Q can fill a cistern in 12 and 15 minutes separately. P is open for 4 minutes and turned off. In how much time will the remaining part be filled by Q?

Two pipes, P and Q, fill a tank in 12 Min and 15 min respectively. That means,

Let total tank capacity 60L

that mean 

P filled tank in 1 minute=5L

Q filled tank in 1 minute=4L

tank filled by P in 4 min is $$=4\times5$$

= 20

Then Pipe P is closed

  So tank Capacity left 60-20=40

i.e filled by Q only

Total time taken by Q to filled remaining part of tank is

$$\frac{total  tank   capacity   left}{Efficiency    of   Q}$$

$$=\frac{40}{4}$$

=10 minutes

So, the time taken by Q is 10 minutes