<s>[INST] The demand function for a certain product is given by $p = 400 - 2x$, where $p$ is the price per unit and $x$ is the number of units sold. The total revenue $R$ is given by the product $px$. Find the marginal revenue function, and use it to determine how the total revenue changes as the number of units sold increases from 0 to 50.[/INST]