A grocery dealer cheats to the extent of 10% while buying as well as selling by using false weight. What is his increase in the profit percentage = ?

Answer: Option B

According to the question,
Cheats while buying = 10%
Cheats while selling = 10%
$$\begin{array}{rl}& \therefore \left(a+b+\frac{ab}{100}\right)\%\\ & =10+10+\frac{10\times 10}{100}\\ & =20+1\\ & =21\end{array}$$
∴ Increase in profit % = 21%