Let $\displaystyle{x}=\usepackage{cmbright}\Huge\$ the cost of 1 book. Then $\displaystyle{4}{x}=\usepackage{cmbright}\Huge\$ the amount spent by Alana, $\displaystyle{2}{x}=\usepackage{cmbright}\Huge\$ the amount spent by Petra, and $\displaystyle{16}{x}-{15}=\usepackage{cmbright}\Huge\$ the amount spent by Angus. Angus spent $5 more than his two friends, so $\displaystyle{16}{x}-{15}={4}{x}+{2}{x}+{5}\usepackage{cmbright}\Huge\$ . Simplify: $\displaystyle{16}{x}-{15}={6}{x}+{5}\usepackage{cmbright}\Huge\$ . Subtract $\displaystyle{6}{x}\usepackage{cmbright}\Huge\$ from both sides: $\displaystyle{10}{x}-{15}={5}\usepackage{cmbright}\Huge\$ . Add 15 to both sides: $\displaystyle{10}{x}={20}\usepackage{cmbright}\Huge\$ . Divide both sides by 10: $\displaystyle{x}={2.00}\usepackage{cmbright}\Huge\$ .