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. At this point, you may have thought Angus's purchase $\displaystyle+\usepackage{cmbright}\Huge\$ 5 = Alana's purchase $\displaystyle+\usepackage{cmbright}\Huge\$ Petra's purchase. However, Angus's purchase = Alana's purchase $\displaystyle+\usepackage{cmbright}\Huge\$ Petra's purchase $\displaystyle+\usepackage{cmbright}\Huge\$ 5, so $\displaystyle{16}{x}-{15}={4}{x}+{2}{x}+{5}\usepackage{cmbright}\Huge\$ .