Question 1153927
This problem may seem puzzling at first since the actual rate of filling, and the actual capacity of the tank are not given. However it can be solved as follows.

Let vol of tank be X litres

Let rate of P1 be y litres/min
Then rate of P2 is {{{3*y}}} litres/min
If they work together, rate is {{{4*y}}} litres /min
and it takes 12 min.
So capacity of tank = {{{4*y * 12 = 48*y}}} litres

So if P1 alone is used, time taken 
= {{{48*y / y = 48 min}}}

If P2 alone is used, time taken
= {{{(48*y) / (3*y) = 16 min}}}
Solved.