Pipes A and B can fill a tank in 30 and 40 minutes, while outlet C empties it in 24 minutes. A and B are opened first, and C is opened 12 minutes later. How long, from the start, does the tank take to fill?

Question

Accepted Answer

30 min. In 12 min A and B fill 12×7/120=7/10. Then the net rate is 1/30+1/40-1/24=4+3-5/120=1/60, so 3/10 takes 18 min. Total 30 min. (18 min is only the last stage.)

Pipes A and B can fill a tank in 30 and 40 minutes, while outlet C empties it in 24 minutes. A and B are opened first, and C is opened 12 minutes later. How long, from the start, does the tank take to fill?

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