Pump X started filling an empty pool with water and continued at a constant rate. 5 hours later, the pool was 1/4 full. Then, pump B joined the work. 9 hours later, the pool was full. Working alone at its constant rate, how long will it take B to fill the empty pool?
(A) 15
(B) 18
(C) 20
(D) 30
(E) 45
answer is D.How do i solve this
(A) 15
(B) 18
(C) 20
(D) 30
(E) 45
answer is D.How do i solve this
