X and Y can do a piece of work in 20 days and 12 days respectively. X started the work alone and then after 4 days .Y joined him till the completion of the work. How long did the work last?
6 days
10 days
15 days
20 days
Explanation:
Work done by X in 4 days = (1/20 - 4) = 1/5
Remaining work = 1 - 1/5 = 4/5
(X+Y)`s 1 day`s work = 1/20 + 1/12 = 8/60 = 2/15
Now, 2/15 work is done by X and Y in 1 day.
So, 4/5 work will be done by X and Y in (15/2 x 4/5) = 6 days.
Hence, total time taken = (6 + 4) days = 10 days.