Answer
a. $26000-400x$
b. $26000x-400x^2$
Work Step by Step
a. We can express the relation between the number of spectators and the ticket price as
$N(x)=20000-400(x-15)=26000-400x$
so that $N(15)=20000$ and $N$ decreases by $400$ for every dollar increase in price.
b. The revenue can be expressed as
$R=Nx=26000x-400x^2$
