If the selling price of 10 pencils is equal to the cost price of 12 pencils then the gain percent is

Let the selling price of each pencil be Rs.1

Then the cost price of 12 pencils $=$ Rs. 10

S.P of 12 pencils $=$ Rs.12

Gain $=S.P-C.P$

$=12-10$

$=2$

Gain $\%=\frac{20}{10}\times 100=20\%$