Rosa can clean a house in 7 hours. When Rosa and Amy work together, they can clean a house in about 4.2 hours. About how long will it take Amy to clean a house if she works by herself?

11 h

10.5 h

10 h

9.5 h

First we will calculate rate of work of rosa and amy.

work done by rosa in time t=t*1/7

Work done by Amy and rosa=t (1/7+1/4.2)

t/7+t/a=t/4.2

t=10.5 hr

Always keep remember those formula or logic below

Work rate * Time to finish the job = 1 job done

Work rate = (1 job done) / (Time to finish the job)

Time of doing the job = (1 job done) / (Work rate)