Work (usually 1 for the whole job) = rate x time.
In this case the rate is the combination of the three people's rates. Sally cleans at 1/8 of the house per hour. Dad cleans at the rate of 1/6 of the house per hour. Dennis the Menace messes (un-cleans) at the rate of 1/10 of the house per hour. So his rate is subtracted when they work together.
1 = (1/8 + 1/6 - 1/10) * t = (23/120)t so t = 120/23 = 5 and 5/23 hours orabout 5 hours 13 minutes.
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