If the selling price of 10 pens is equal to the cost price of 15 pens, then the gain per cent is:

Correct option is D

Given:

Selling price (SP) of 10 pens = Cost price (CP) of 15 pens  

Formula Used:  

Gain % = $$\left( \frac{SP-CP}{CP} \times 100 \right)$$   

Solution:    

As given ;

SP of 10 pens = CP of 15 Pen 

So, $$\frac{SP}{CP} = \frac{15}{10} = \frac{3}{2}$$   

Let SP = 3x  and CP = 2x 

So, Gain % = $$\left( \frac{3x - 2x}{2x} \times 100 \right) = \frac{1}{2}\times 100 = 50 \%$$  ​