Substitute 5.1 for R in the formula: $\displaystyle{5.1}={\log{{\left(\frac{{A}}{{A}_{{0}}}\right)}}}\usepackage{cmbright}\Huge\$

Rewrite as an exponential equation: $\displaystyle{{10}}^{{5.1}}=\frac{{A}}{{A}_{{0}}}\usepackage{cmbright}\Huge\$

Use a calculator and multiply both sides by $\displaystyle{A}_{{0}}\usepackage{cmbright}\Huge\$ : $\displaystyle{A}\approx{125},{893}{A}_{{0}}\usepackage{cmbright}\Huge\$