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.