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?

Question

I did at first try the problem out but i am not too sure about the answer since i got like a fractional value as the answer. I didn't know where to corporate the "4 days" condition to be honest and thought it wasn't needed but i am not sure yet again. Thank you.

Answer 1

Work done by x in a day=1/20

Work done by x in 4 days=4(1/20)=1/5

Work remaining=1-1/5=4/5

Work done by x and y in a day=(1/20  +  1/12)= 8/60=2/15

days:work done::days:work done

1:2/15::x:4/5

x=(4/5)(15/2)=6

Both of them completed the remaining work in 6 days.

Total number of days =4+6=10