Find the selling price if the cost price is $360 and the gain is 12.5% on cost

It is important to note the details already given in the question. Based on those details the answer to the question can be easily determined. Now let us focus on the question given.
Cost price of the product in question = $360
Percentage of gain on the product = 12.5%
Then
amount of gain made on the product = (12.5/100) * 360 dollars
                                                           = 450/10 dollars
                                                           = 45 dollars
So
The selling price of the product in question = (360 + 45) dollars
                                                                     = 405 dollars