A water tank can be filled by two inlets, labeled A and B, in 12 hours and 15 hours respectively. An outlet pipe C can empty the tank in 10 hours. If all three pipes are opened together, how much time will it take to fill the tank?
Show answer & explanation
Correct answer: C
Explanation
Rate of A = 1/12 tank/hour
Rate of B = 1/15 tank/hour
C (emptying rate) = −1/10 tank/hour
Net rate = 1/12 + 1/15 − 1/10
= 5/60 + 4/60 − 6/60
= 3/60 = 1/20
Time to fill completely = 20 hours