Question

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?

Answer 1

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.