You may have chosen this approximation because it falls between 7 and 8 and you know that $\displaystyle\sqrt{{{54}}}\usepackage{cmbright}\Huge\$ is between $\displaystyle\sqrt{{{49}}}\usepackage{cmbright}\Huge\$ and $\displaystyle\sqrt{{{64}}}\usepackage{cmbright}\Huge\$ . However, because 54 is closer to 49 than it is to 64, the approximation will be closer to 7 than to 8. Using trial and error, square different approximations to find the best estimate of $\displaystyle\sqrt{{{54}}}\usepackage{cmbright}\Huge\$ .