Marginal Revenue Questions

The per-unit profit of a product is given by $P=600-5x$, where $x$ is the number of units sold. What is the expression for marginal revenue?

A manufacturing firm sets the price per unit as $p(x)=400-\alpha x^2$, where $\alpha$ is a constant and $x$ is the units demanded. If the marginal revenue when 10 units were in demand was zero, then the value of $\alpha$ is:

A manufacturing firm has its variable cost given by $C(x)=x(12-x^3)$. The price per unit is $p(x)=6x-2$. Find the marginal revenue when $3$ units were in demand.