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\$ . Substituting the given numbers:

$\displaystyle{42.40}={k}{\left({20}\right)}{\left({2}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{42.40}={40}{k}\usepackage{cmbright}\Huge\$

$\displaystyle{k}={1.06}\usepackage{cmbright}\Huge\$