Explanation:

Solution: 
100 men completed 5/12 work in 35 days
So 100 men can complete the remaining 7/12 work in 49 days:
Use M1*D1*W2 = M2*D2*W1
100*35*(7/12) = 100*D2* (5/12)
D2 = 49 days
But after 35 days, 40 more men were employed, so 140 men now and they completed 7/12 work in
By M1*D1*W2 = M2*D2*W1
100*35*(7/12) = 140*D2*(5/12)
D2 = 35 days
So extra days = 49-35 = 14 days