assuming the house is clean when Mimi gets home, consider that in one hour,
1/5 - 1/7 = 2/35 of the house gets messed up each hour.
Suppose that Hortense and Mort can clean their entire house in 7 hours, while their toddler, Mimi, just by being around, can completely mess it up in only 5 hours. If Hortense and Mort clean house while Mimi is at her grandmother's, and then start cleaning up after Mimi the minute she gets home, how long does it take from the time Mimi gets home until the whole place is a shambles?
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