suppose a firm lowers its price to 10 raising the equanitity

suppose a firm lowers its price to $10, raising the equanitity sold from 4 to 5 units. If the marginal revenue of the fifith unit is $2, the firm must have lowered its price by:

I know the answer is $2, but not sure how.

Solution

Suppose the previous price was p

Then, the change in the revenue(marginal revenue) is equal to 5x10 - 4xp =50-4p

It is given that 50-4p=2

So, p=12

Thus, the firm has lowered the price by 12-10=$2