A does 20% less work than B. If A can complete a piece of work in 71/2 hours, then B can do it in ?

Answer: Option C

Ratio of times taken by A and B
$$\begin{array}{rl}& =100:80\\ & =5:4\end{array}$$
Suppose B takes x hours to do the work
$$\begin{array}{rl}& \Rightarrow 5:4::\frac{15}{2}:x\\ & \Rightarrow x=\left(\frac{4\times 15}{2\times 5}\right)\\ & \Rightarrow x=6\text{ hours}\end{array}$$