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:

Question: 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:

Solution:
The work rate of A is 1/15 (since A can complete the work in 15 days).
The work rate of B is 1/20​ (since B can complete the work in 20 days).

In 4 days they complete = 4{(1/15) + (1/20)}
= 4 × 7/60
= 7/15

then the fraction of the work that is left is = 1 - (7/15)
= (15 - 7)/15
= 8/15