Medium
Pipe A can fill a tank in 10 hours, and pipe B can empty the full tank in 15 hours. If both pipes are opened together, in how many hours will the empty tank be filled?
Explanation
Explanation
Step 1: Work done in 1 hour: pipe A fills 1/10 of the tank, and pipe B empties 1/15 of the tank.
Step 2: Net filling in 1 hour = 1/10 − 1/15 = 3/30 − 2/30 = 1/30.
Step 3: If 1/30 of the tank fills in 1 hour, the whole tank fills in 30 hours.
Step 4: Check: in 30 hours, A fills 3 tanks and B empties 2 tanks, so the net gain is exactly 1 tank ✓.
Answer: 30 hours.
PRO TIP
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