Explanation:

Solution: 
LCM = 120
A = 40………..120/40= 3
B = 60…………………… 2
.                                    5
12*5= 60
Remaining 60 will complete in 6 days. So 60/6 = 10. Now this 10 work will be complete by together .
Child do 5 work in a day . now the ratio of the total work of all them.
A ……………..B……………. C
3*18 ……..2*18…………. 5*6
54…………. 36……………. 30
9 : 6 : 5
Child – 5/20*2000= 500