Quantitative Aptitude :: Time and Work - Discussion

A can do 4/5th works in 20 days.
so he can do the whole work in 20*(5/4) =25 days.
remaining work after done 4/5 by A is=1-(4/5)=1/5
(A+B) can do the 1/5th work in 3 days.
means the can do the whole work in (3*5)=15 days.
so,(A+B)'s 1 days work=1/15
B's 1 day's work {(1/15)-(1/25)}=2/75
so B can alone do the whole work in 75/2,means in 37 1/2 days.

In two days A, B and C together can finish 1/2 of a work and in another 2 days B and C together can finish 3/10 part of the work. Then A alone can complete the whole work in.