What is the formula to calculate Profit Percentage? Let cost price of an item be $C$, selling price be $S$. Assume the seller makes a profit. 

Then profit would be: $P = S - C$.

Now, what is the formula for calculating Profit Percentage?


$P \% = \dfrac{P}{C} \times 100$
$P\% = \dfrac{P}{S} \times 100$


Which one is right and why?
They are both right.

The first expresses the profit as a percentage of the cost price. This is the profit mark-up.

The second expresses the profit as a percentage of the sales price. This is the profit margin.

Percentage profit on its own doesn't mean anything unless you are talking mark-up or margin. By convention, in a module I teach, percentage profit means profit mark-up.
answer two is right because :
Suppose
Cost of your product is 5400
Selling price is 8000
then profit is 2600

According to 2nd formula P%=P/S×100
profit % = 2600/8000 X 100 = 32.5%

Let's check 
profit according to 32.5%

profit = Selling price X profit % /100
         8000 X 32.5% / 100 = 2600

hence formula 2nd is right