2 men and 7 boys can do a piece of work in 4 days.

Solution:

A man can alone finish the work in $x$ days and one boy alone can finish it in $y$ days then

One mans one days work $=\frac{1}{x}$

One boys one days work $=\frac{1}{y}$

2 men one day work $=\frac{2}{x}$

7boys one day work $=\frac{7}{y}$

Since 2 men and 7 boys can finish the work in 4 days

$4\left(\frac{2}{x}+\frac{7}{y}\right)=1$

$\frac{8}{x}+\frac{28}{y}=1 \cdots(i)$

Again 4 men and 4 boys can finish the work in 3 days

$3\left(\frac{4}{x}+\frac{4}{y}\right)=1$

$\frac{12}{x}+\frac{12}{y}=1 \cdots(i i)$

Putting $\frac{1}{x}=u$ and $\frac{1}{y}=v$ in equation $(i)$ and $(i i)$ we get

$8 u+28 v=1$

$12 u+12 v=1$

$8 u+28 v-1 \cdots($ iii $)$

$12 u+12 v-1 \cdots(i v)$

By using cross multiplication we have

Now,

$u=\frac{1}{15}$

$\frac{1}{x}=\frac{1}{15}$

$x=15$

and

$v=\frac{1}{60}$

$\frac{1}{y}=\frac{1}{60}$

$y=60$

Hence, one man alone can finish the work in 15 days and one boy alone can finish the work in 60 days