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Answer:
10
Step-by-step explanation:
Rate of drainage for pump A and B = [1 pool drained]/[6 hours]
Rate of drainage for pump A (older pump) = [1 pool drained]/[15 hours]
Rate for pump B (newer pump) = [1 pool drained]/[t hours]
Thus, adding the rate of drainage of A and B,
1/t + 1/15 = 1/6
t=10