Five drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 16 days. How many additional pipes, each draining water at the same constant rate, will be needed to drain the pool in 4 days?
Choices:
A) 6	B) 9	C) 10	D) 12	E) 15
This is an inverse proportional problem......   5 pipes in 16 days; So for 4 days, it will be = 16 x 5 / 4 = 20   So, 20-5 = 15   Answer 15
<result>E</result>
Five drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 16 days. How many additional pipes, each draining water at the same constant rate, will be needed to drain the pool in 4 days?