If the group is exactly 20, the cost is $100 per person. The company offers to discount each person's ticket by 5 dollars for each addition above. If x is the number of additional people above 20, then the revenue is given by R(x)=(number of people)(ticket numbers)=_________________________ x ______________________ dollars.

The marginal revenue function is R'(x)=___________________ dollars/person over 25) write in b*c form

I have attempted this questions by stating that the revenue function is (25+x)(100-5x) and I took the derivative and got -10x-25; however, I don't know I am doing the question write because the marginal revenue function is asking to write in b*c which i also don't understand.

Could someone tell me I am going on the right track if so what do I do after I have the the equation with the revenue function. If i am not on the right track can someone direct me to the way this questions should be answered. Thanks