You may have thought that the $24 tax was on the $350, then found the tax on $400, and rounded to the nearest dollar. Because this is a direct variation problem, use the equation $\displaystyle{y}={k}{x}\usepackage{cmbright}\Huge\$ , where y is the sales tax and x is the cost. Substitute $\displaystyle{y}={24}\usepackage{cmbright}\Huge\$ and $\displaystyle{x}={400}\usepackage{cmbright}\Huge\$ : $\displaystyle{24}={k}{\left({400}\right)}\usepackage{cmbright}\Huge\$ . Solve for k. Then use this value of k and $\displaystyle{x}={350}\usepackage{cmbright}\Huge\$ in a new direct variation equation, and solve for y. The correct answer is $21.