You may have left off the 4 at the end of the decimal. You need to simplify $\displaystyle\frac{{644}}{{1000}}\usepackage{cmbright}\Huge\$ , not $\displaystyle\frac{{64}}{{100}}\usepackage{cmbright}\Huge\$ . Because 644 is divisible by 4, the fraction simplifies to $\displaystyle\frac{{161}}{{250}}\usepackage{cmbright}\Huge\$ , so that is the correct answer.