Two pipes can fill a cistern in 10 hours and 15 hours respectively, while a third drain pipe can empty the full cistern in 30 hours. If all three pipes are opened simultaneously, in how many hours will the empty cistern be completely filled?
Answer and explanation
Correct answer: D.
- Step 1: Determine the net hourly filling rate: Inlet A fills per hour, Inlet B fills per hour, and Drain C empties per hour.
- Step 2: Net rate . Find common denominator (30): of the tank per hour. Time taken to fill .
Answer
.
Why not the others: '' ignores pipe B; '' approximates as 8; '' uses an incorrect drain rate.
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