If the price of erasers goes down by 25%, a man can buy 2 more erasers for a rupee. How many erasers are available for a rupee ?

Let the original price of an eraser be Rs. x
Reduced price = 75% of Rs. x = Rs. $$\frac{3x}{4}$$
$$\eqalign{ & \therefore \frac{1}{{\left( {\frac{{3x}}{4}} \right)}} - x = 2 \cr & \Rightarrow \frac{4}{{3x}} - \frac{1}{x} = 2 \cr & \Rightarrow \frac{1}{{3x}} = 2 \cr & \Rightarrow 6x = 1 \cr & \Rightarrow x = \frac{1}{6} \cr} $$
Hence, number of erasers available for a rupee = 6