By selling 90 ball pens for Rs. 160, a person loses 20% . How many ball pens should be sold for Rs. 96 so as to have a profit of 20% ?

Answer: Option B

$$\begin{array}{rl}& \text{S}\text{.P}\text{. of 90 ball pens}\\ & =\text{Rs}\text{. 160, loss}=20\%\\ & \text{C}\text{.P}\text{.of 90 ball pens}\\ & =\text{Rs}.\left(\frac{100}{80}\times 160\right)\\ & =\text{Rs}\text{. }200\end{array}$$
∴ Desired S.P. of 90 ball pens
$$\begin{array}{rl}& =\text{Rs}.\left(\frac{120}{100}\times 200\right)\\ & =\text{Rs}\text{. }240\end{array}$$
For Rs. 240, ball pens sold = 90
For Rs. 96, ball pens sold
$$\begin{array}{rl}& =\left(\frac{90}{240}\times 96\right)\\ & =36\end{array}$$