New memberships at a local gym are priced at $100 for adults and $75 for students. During the past week 70 memberships were sold and a total of $6,300 was collected. This can be described by a system of equations and by a graph:

$\displaystyle{100}{A}+{75}{S}={6},{300}\usepackage{cmbright}\Huge\$ and $\displaystyle{A}+{S}={70}\usepackage{cmbright}\Huge\$

How many adult memberships were sold?