Answer
$h=149.102$ hr
Work Step by Step
$m(t)=m_02^{-t/h}$. Whereas,$m(t)$ is the mass of radioactive substance after time $t$, $m_0$ is the Initial mass of radioactive substance, $h$ is the half-life and $t$ is time.
$t=48$ hr, $m(t)=200$, $m_0=250$,
$\frac{m(t)}{m_0}=2^{-t/h}$,
$\log (\frac{m(t)}{m_0})=-t/h \log2$,
$h=\frac{t\log2}{\log(\frac{m(t)}{m_0})}=149.102$ hr
