You may have incorrectly substituted into the formula and gotten $\displaystyle{P}={2}\cdot{10}+{15}\usepackage{cmbright}\Huge\$ . You are given that $\displaystyle{a}={15}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={10}\usepackage{cmbright}\Huge\$ , so the correct substitution would be $\displaystyle{P}={2}\cdot{15}+{10}\usepackage{cmbright}\Huge\$ .