Question

It takes Trevon ten hours to clean an attic.
Cody can clean the same attic in seven
hours. Find how long it would take them if
they worked together

Answer 1

Trevon takes 10 hours, so every hour he cleans 1/10th of the attic (or 0.1 out of 1)
Similarly Codey cleans 1/7th of the attic per hour (0.14286ish out of 1)
So if they are both cleaning, the attic is being cleaned at a rate of 1/10th + 1/7th per hour = 0.1+0.14286=0.24286 per hour. We want to find how many hours it takes until the attic is cleaned. Call t the number of hours, then after t hours, t*0.24286 of the attic has been cleaned. So t*0.24846=1, and so t=1/(0.24846)=4.118 = 4 hours and 0.118 of an hour = 4 hours and 7 minutes.
Sorry if this isn't very clear... I'm not great at explaining things :")