Question

If 3 men or 6 boys can complete a work in 40 days, in how many days,6 men and 8 boys will complete the same work?
A. 10
B.11
C.12
D.4

Answer 1

3 men can complete a work (amount = W) in 40 days. So,
1 man can complete W in 40 * 3 days = 120 days.
Work per day of 1 man = capacity of 1 man = $\frac{1}{120}$ of W per day

6 boys can complete W in 40 days. Similarly, we get,
work per day of 1 boy = $\frac{1}{240}$ of W

Work per day by 6 men and 8 boys =
    $\frac{6}{120}\ W + \frac{8}{ 240}\ W\ = \frac{20}{240}\ of\ W = \frac{1}{12}$ of W

So, 6 men and 8 boys can complete work W in : $\frac{W}{\frac{1}{12} W}$ = 12 days