**Question:**

Mohan takes 9 hours to mow a large lawn. He and Sohan together can mow it in 4 hours. How long will Sohan take to mow the lawn if he works alone?

**Solution:**

Time taken by Mohan to do the work $=9$ hours

Time taken by Mohan and Sohan to do the work $=4$ hours

$\therefore$ Work done by Mohan $=\frac{1}{9}$

Work done by Mohan and Sohan $=\frac{1}{4}$

$\therefore$ Work done by Sohan $=\frac{1}{4}-\frac{1}{9}$

$=\frac{9-4}{36}=\frac{5}{36}$

Thus, Sohan can do the work in $\frac{36}{5}$ hours i. e. $7 \frac{1}{5}$ hours.