1. ## Word Problem

A man can do a certain task in 21 hours, another can do the task in 28 hours, and a boy can do the task in 48 hours. How long will it take to do the task if all work together?

2. Every hour man 1 does $\frac{1}{21}$ of the job, man 2 does $\frac{1}{28}$ of the job, and the boy does $\frac{1}{48}$ of it.

So that means, every hour $\frac{1}{21} + \frac{1}{28} + \frac{1}{48}$ of the job gets done

$= \frac{5}{48}$.

So if we set up a ratio (amount of job : hours)...

$\frac{5}{48} : 1$

$5 : 48$

$1 : \frac{48}{5}$

$1 : 9.6$.

So they will finish the job in $9.6$ hours, or 9 hours 36 mins.