From 2008 to 2009, the sales of a book decreased by 80%. If the sales in 2010 were the same as in 2008, by what percent did it increase from 2009 to 2010 ?

Answer: Option B

Decrease in sale = 80% from 2008 to 2009
Now, sales of 2010 are same as in 2008
:. Increase in sales
$$\begin{array}{rl}& =\left[\frac{100\times \text{a}}{100-\text{a}}\right]\%\\ & =\left[\frac{100\times 80}{100-80}\right]\%\left[\text{a}=80\%\right]\\ & =\frac{8000}{20}\%\\ & =400\%\end{array}$$