Answer
$t=18.03$
Work Step by Step
$m(t)=m_0e^{-rt}$. Whereas,$m(t)$ is the mass of radioactive substance after time $t$, $m_0$ is the Initial mass of radioactive substance, $r=\frac{\ln 2}{h}$ is the rate of decay while $h$ is the half-life and $t$ is time.
$r=\frac{\ln2}{h}=0.024755$,
$m(t)=m_0e^{-rt}=50e^{-0.024755t}=32$,
$e^{-0.024755t}=0.64$,
$-0.024755t=\ln 0.64$,
$t=18.03$
