One way to subtract is to regroup one of the whole numbers: $\displaystyle{6}={5}\frac{{6}}{{6}}\usepackage{cmbright}\Huge\$ . Then subtract as with any other mixed numbers: $\displaystyle{5}\frac{{6}}{{6}}-{1}\frac{{5}}{{6}}={4}\frac{{1}}{{6}}\usepackage{cmbright}\Huge\$ .