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

Simple Maths,

suppose total work is1

A=1/15 & B=1/20 for one day work

when both work together

(A+B)= (1/15)+(1/20)=7/60

for4 days

(7/60)x4=7/15

for remaining work

1-(7/15)=8/15

Answer (8/15)