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Working alone, Lia can mow a sports field in 3 hours, and working alone, Theo can mow the same field in 6 hours. If they work together at their individual constant rates, how many hours will it take them to mow the field?
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Correct answer
B
Add rates, not times. Lia's rate is 1/3 of the field per hour; Theo's is 1/6 per hour. Combined rate = 1/3 + 1/6 = 2/6 + 1/6 = 3/6 = 1/2 field per hour.
Time = 1 / (1/2) = 2 hours. Note the result is less than either solo time, which is the sanity check.
Averaging the times (4.5) or adding them (9) both ignore that they work at the same time.
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