Single correct option
Pipes A and B can fill an empty tank in 20 minutes and 30 minutes respectively, and outlet C can empty the full tank in 15 minutes. All three are opened together on the empty tank, and after 10 minutes outlet C is closed. How many more minutes will it take to fill the tank?
- A. 8
- B. 10
- C. 12
- D. 15
Show answer and solution
Answer: (B) 10
Take tank capacity = 60 units (LCM of 20, 30, 15). A fills 3 units/min, B fills 2 units/min, C empties 4 units/min.
With all three open, net = 3 + 2 − 4 = 1 unit/min. In 10 minutes, 10 units are filled.
Remaining = 50 units; with only A and B open, rate = 5 units/min, time = 50/5 = 10 minutes.
Why 12 is wrong: it comes from using the full 60 units after C closes, ignoring the 10 units already filled.
With all three open, net = 3 + 2 − 4 = 1 unit/min. In 10 minutes, 10 units are filled.
Remaining = 50 units; with only A and B open, rate = 5 units/min, time = 50/5 = 10 minutes.
Why 12 is wrong: it comes from using the full 60 units after C closes, ignoring the 10 units already filled.
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