- A8.75
- B18
- C6
- D10.5
Step-by-Step Solution
Combined rate = 1/15 + 1/21 = 7/105 + 5/105 = 12/105, so time = 105/12 = 8.75 hours.
Answer: A) 8.75
An overhead tank has two inlet pipes that fill it in 15 hours and 21 hours respectively. With both pipes open, how many hours does it take to fill the empty tank?
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