More workers, Less time needed

First, we need to find out the productivity (how much work they can do at a time) of 4 boys which is: $\frac{1}{Time}$ = $\frac{1}{10}$

Now we need to find productivity of 1 boy which is: $\frac{1}{10}$ X $\frac{1}{4}$ = $\frac{1}{40}$

Productivity of 3 boys: $\frac{1}{40}$ X 3 = $\frac{3}{40}$

Work done by 4 boys in 3 days: $\frac{1}{10}$ X 3 = $\frac{3}{10}$

Remaining work: 1 - $\frac{3}{10}$ = $\frac{7}{10}$

Remaining time to complete rest of the work: $\frac{7}{10}$ X $\frac{40}{3}$ = $\frac{28}{3}$