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)

Iam guessing c. or d.