The initial amount $\displaystyle{A}={120}\usepackage{cmbright}\Huge\$ , the time $\displaystyle{t}={24}\usepackage{cmbright}\Huge\$ , and the half-life $\displaystyle{h}={8}\usepackage{cmbright}\Huge\$ . Substitute into the formula and simplify:

$\displaystyle{R}={120}{{\left(\frac{{1}}{{2}}\right)}}^{{\frac{{24}}{{8}}}}={120}{{\left(\frac{{1}}{{2}}\right)}}^{{3}}={120}{\left(\frac{{1}}{{8}}\right)}={15}\usepackage{cmbright}\Huge\$