Let $\displaystyle{x}=\usepackage{cmbright}\Huge\$ the cost of 1 shirt. You may have applied the discount to each shirt that Kiyoshi bought, instead of to the whole purchase. That is, you thought he spent $\displaystyle{6}{\left({x}-{30}\right)}\usepackage{cmbright}\Huge\$ dollars, when he actually spent $\displaystyle{6}{x}-{30}\usepackage{cmbright}\Huge\$ dollars. The correct answer is $25.