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 B

Let time taken by B = x
$$\text{Efficiency}\propto \frac{1}{\text{Time}\text{taken}}$$
So, if B is 100% efficient. then A is 80% efficient
So,
$$\begin{array}{rl}& \Rightarrow \frac{80}{100}=\frac{x}{15/2}\\ & \Rightarrow x=\frac{80\times 15}{100\times 2}\\ & \Rightarrow x=6\text{hours}\end{array}$$

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Alternate:
Number of hours taken by A to finish the work = $$7\frac{1}{2}$$ hours = $$\frac{15}{2}$$ hours
Work done by A in one hour : $$\frac{2}{15}$$
Let number of hours taken by B to finish the work : $$\frac{1}{\text{x}}$$
A can work 20% less than B that is $$\frac{20}{100}$$  = $$\frac{4}{5}$$ times of B’s work.
$$\begin{array}{rl}& \text{Here,}\frac{4}{5}:1=\frac{2}{15}:\frac{1}{\text{x}}\\ & \frac{4}{5}=\frac{2\text{x}}{15}\\ & \text{x}=\frac{15\times 4}{5\times 2}=6\text{hours}\end{array}$$