Substitute 4.9 for R in the formula: $\displaystyle{4.9}={\log{{\left(\frac{{A}}{{A}_{{0}}}\right)}}}\usepackage{cmbright}\Huge\$

Rewrite as an exponential equation: $\displaystyle{{10}}^{{4.9}}=\frac{{A}}{{A}_{{0}}}\usepackage{cmbright}\Huge\$

Use a calculator and multiply both sides by $\displaystyle{A}_{{0}}\usepackage{cmbright}\Huge\$ : $\displaystyle{A}\approx{79},{433}{A}_{{0}}\usepackage{cmbright}\Huge\$