Explanation:

A can do a work in 15 days and B in 20 days. If they together work on it for 4 days, then the fraction of the wok that is left is

total work=lcm(15,20)=60 unit

work done in 4 days=4 x 7=28

remaing work=60-28=32

fraction of work left=32/60=8/15