How to determine the selling price that will maximise the revenue?

Question

Funnel Cakes cost $10. In one hour, an average of 40 funnel cakes are sold. For every dollar increase in price, there will be 2 fewer funnel cakes sold. Let x represent the number of increases. Let R represent the total revenue. R(x) = (10 + x)(40 – 2x)

Answer 1

Revenue reaches maximum for the price $15 (and  quantity 30).How to get this just maximize Revenue function.