### The cost price of an article is 80% of its marked price for sale. How much per cent does the tradesman gain after allowing a discount of 12%?

A. Rs. 70 B. Rs. 80 C. Rs. 90 D. Rs. 100 Answer: Option C
$\begin{array}{l}\text{Let the original price of tea be Rs. x kg}\\ \text{After reduction of the price becomes }\\ \text{= x – 10% of x = }\frac{9x}{10}\ \text{per kg}\\ \text{Now,}\\ \frac{22500}{\frac{9x}{10}}-\frac{22500}{x}=25\\ \text{or,}\ 22500\left[\frac{10}{9x}-\frac{1}{x}\right]=25\\ \text{or,}\ 25\times9x=22500\\ \text{or,}\ x=\frac{22500}{25\times9}=\text{Rs.}\ 100\\ \text{Hence, new price }\\ \text{= 90 per kg}\end{array}$