Answer
$130$ seconds
Work Step by Step
Use the exponential decay model
$m(t)=m_0\cdot 2^{-t/h}$
and the given conditions:
$h=30, m(t)=(1-0.95)m_0$
$0.05m_0=m_0\cdot 2^{-t/30}$
which gives
$0.05=2^{-t/30}$
$2^{t/30}=20$
$t=30\times \log_2 20\approx130$ seconds