Let $\displaystyle{x}=\usepackage{cmbright}\Huge\$ the cost of 1 book. Then $\displaystyle{4}{x}=\usepackage{cmbright}\Huge\$ the amount spent by Alana, $\displaystyle{5}{x}=\usepackage{cmbright}\Huge\$ the amount spent by Petra, and $\displaystyle{19}{x}-{10}=\usepackage{cmbright}\Huge\$ the amount spent by Angus. Angus spent $7.50 more than his two friends, so $\displaystyle{19}{x}-{10}={5}{x}+{4}{x}+{7.50}\usepackage{cmbright}\Huge\$ . Simplify: $\displaystyle{19}{x}-{10}={9}{x}+{7.50}\usepackage{cmbright}\Huge\$ . Subtract $\displaystyle{9}{x}\usepackage{cmbright}\Huge\$ from both sides: $\displaystyle{10}{x}-{10}={7.50}\usepackage{cmbright}\Huge\$ . Add 10 to both sides: $\displaystyle{10}{x}={17.50}\usepackage{cmbright}\Huge\$ . Divide both sides by 10: $\displaystyle{x}={1.75}\usepackage{cmbright}\Huge\$ .