Let $\displaystyle{x}=\usepackage{cmbright}\Huge\$ the cost of 1 shirt. Then Kiyoshi spends $\displaystyle{6}{x}-{30}\usepackage{cmbright}\Huge\$ dollars, and Gregory spends $\displaystyle{4}{x}\usepackage{cmbright}\Huge\$ dollars. Gregory spends $20 less than Kiyoshi, so $\displaystyle{4}{x}={6}{x}-{30}-{20}\usepackage{cmbright}\Huge\$ . Simplify: $\displaystyle{4}{x}={6}{x}-{50}\usepackage{cmbright}\Huge\$ . Subtract $\displaystyle{6}{x}\usepackage{cmbright}\Huge\$ from both sides: $\displaystyle-{2}{x}=-{50}\usepackage{cmbright}\Huge\$ . Divide both sides by $\displaystyle-{2}\usepackage{cmbright}\Huge\$ : $\displaystyle{x}={25}\usepackage{cmbright}\Huge\$ .