Substitute 4.2 for R in the formula: $\displaystyle{4.2}={\log{{\left(\frac{{A}}{{A}_{{0}}}\right)}}}\usepackage{cmbright}\Huge\$

Rewrite as an exponential equation: $\displaystyle{{10}}^{{4.2}}=\frac{{A}}{{A}_{{0}}}\usepackage{cmbright}\Huge\$

Use a calculator and multiply both sides by $\displaystyle{A}_{{0}}\usepackage{cmbright}\Huge\$ : $\displaystyle{A}={15},{849}{A}_{{0}}\usepackage{cmbright}\Huge\$