i need help thanks!!!!!!!!!!!!!!!!!!!!!!!!!!!!

i need help thanks!!!!!!!!!!!!!!!!!!!!!!!!!!!!

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 :

A.
1
4
B.
1
10
C.
7
15
D.
8
15