A pond has an inlet to fill it up and an outlet to drain it. The inlet can fill it in 5 hours and the the outlet can drain it in 8 hours. Suppose the pond was empty and the person in charge of filling it up opened the inlet but forgot to close the outlet. After 4 hours, he realized that the outlet is still open. At that time, he closed the outlet. How long did it take for the pond to get filled after the outlet was closed?
C? this question was kinda confusing cause I thought it was asking for the total time.
Anyway. 4*1/5 = 4/5 4*1/8 ---> 1/2 --> 4/5-1/2=3/10
so we have 7/10 remaining.
Now its just 7/10/1/5 --> 35/10 = 3.5