Question

The problem is 

Answer 1

$average=\frac{sum\ of\ attendance}{amount\ of\ shows}\\\\ average=3000\\\\ 3000=\frac{2580+2920+third\ show}{3}\ \ \ |*3\\ 9000=2580+2920+third\ show\\ 9000=5500+third\ show\ \ \ |Subtract\ 5500\\ 3500=third\ show\\\\ \textbf{3500\ people\ must\ attend\ the\ third\ show.}$