Let total work be W
Work done by 1 man in 1 month = 10∗7W=70W
Work done by 1 woman in 1 month = 20∗7W=140W
Work done by 1 child in 1 month = 40∗7W=280W
Total work done by 5 men, 5 women and 5 children in 1 month= 705W+1405W+2805W=8W
Let number of months be n
8W∗n=2W
n=4