Five drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 12 days. How many additional pipes K, each draining water at the same constant rate, will be needed to drain the pool in 4 days? Choose the most appropriate option.
A) 6
B) 9
C) 10
D) 12
E) 15
This is an inverse proportional problem......   5 pipes in 12 days; So for 4 days, it will be = 12 x 5 / 4 = 15   So, K=15-5 = 10.10
<result>C</result>
Five drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 12 days. How many additional pipes K, each draining water at the same constant rate, will be needed to drain the pool in 4 days?