Answer
$t=129.63$ seconds
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.
$r=\frac{\ln2}{h}=0.02311$
$m(t)=m_0e^{-rt}=1e^{-0.02311t}=0.05$,
$e^{-0.02311t}=0.05$,
$-0.02311t=\ln 0.05$,
$t=129.63$ seconds
