Answer
$78s$
Work Step by Step
We can find the required half-life as follows:
$T_{\frac{1}{2}}=\frac{\ln 2}{\lambda}$
We plug in the known values to obtain:
$T_{\frac{1}{2}}=\frac{\ln 2}{8.9\times 10^{-3}}$
$T_{\frac{1}{2}}=78s$
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