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)
