You may just have multiplied 2 and 20 and subtracted that from the total. If T is the total paid, n is the number of items, and p is the price of one item, then $\displaystyle{T}={k}{n}{p}\usepackage{cmbright}\Huge\$ . Substitute the given numbers, $\displaystyle{T}={42.40}\usepackage{cmbright}\Huge\$ , $\displaystyle{n}={20}\usepackage{cmbright}\Huge\$ , and $\displaystyle{p}={2}\usepackage{cmbright}\Huge\$ , and solve for k.