If the cost price of 12 pens is equal to the selling price of 8 pens, the gain percent is ?

Friends, we know we will need gain amount to get gain percent, right. So lets get gain first.
Let the cost price of 1 pen is Re 1

Cost of 8 pens = Rs 8

Selling price of 8 pens = 12

Gain = 12 - 8 = 4

Gain% = $\frac{gain}{\cos t}\times 100$%  =  $\frac{4}{8}\times 100$% = 50%