Explanation:

A, B, and C complete work in 10,12 and 15 days

3 days work

3(1/12  +1/10  +1/15)

(1/4 +3/10 +1/5)

(5+6+4) /20

15/20

=3/4

Remaining work 1/4

X(1/12 +1/15) =1/4

X(9/60) =1/4

X=15/9 days

Total work=3+5/3

           =14/3 days