If cost price of 12 books is equal to the selling price of 10 books, then what is the profit percent?

Correct option is D

Given:
Cost price of 12 books = Selling price of 10 books.
Formula Used:
​$$\text{Profit\%} = \left(\frac{\text{SP} - \text{CP}}{\text{CP}}\right) \times 100$$​​

Solution:
Let CP of 1 book be Rs. x.

Then, CP of 12 books = 12x and SP of 10 books = 12x.

Therefore, SP of 1 book = 12x / 10 = 1.2x

Profit per book = SP - CP = 1.2x - x = 0.2x
​$$\text{Profit\%} = \left(\frac{0.2x}{x}\right) \times 100 = 20\%$$

Alternate Solution:

Cost price of 12 books = Selling price of 10 books.

12 C.P. = 10 S.P. 

​$$\frac{\text{C.P.}}{\text{S.P.}} = \frac{10}{12} = \frac{5}{6}$$
$$\text{Profit\%} = \left(\frac{\text{SP} - \text{CP}}{\text{CP}}\right) \times 100$$

$$\text{Profit\%} = \left(\frac{\text{6} - \text{5}}{\text{5}}\right) \times 100 = 20\%$$​​​