Answer
a. $40000-500x$
b. $40000x-500x^2$
Work Step by Step
a. We can express the relation between the number of spectators and the ticket price as
$N(x)=30000-500(x-20)=40000-500x$
so that $N(20)=30000$ and $N$ decreases by $500$ for every dollar increase in price.
b. The revenue can be expressed as $R=Nx=40000x-500x^2$
