Do not forget to subtract 1 from the first number when you regroup. You need to subtract $\displaystyle{5}\frac{{6}}{{6}}-{1}\frac{{5}}{{6}}\usepackage{cmbright}\Huge\$ , not $\displaystyle{6}\frac{{6}}{{6}}-{1}\frac{{5}}{{6}}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{4}\frac{{1}}{{6}}\usepackage{cmbright}\Huge\$ .