The pool can be filled with the first pump in 4 hours or with the second pump in 12 hours How many hours an empty pool

The pool can be filled with the first pump in 4 hours or with the second pump in 12 hours How many hours an empty pool will be filled if both pumps are turned on simultaneously.

The first pump fills 1/4 of the pool in an hour, the second in the same time 1/12 of the pool. If you turn them on together in one hour, they fill:
(1/4) + (1/12) = (3/12) + (1/12) = 4/12 = 1/3 pool.
This means that to completely fill the pool you will need:
1 / (1/3) = 3 h.
Answer: with two simultaneously switched on pumps, the pool will be filled with water in 3 hours.