Answer
The average power output of this magnetar was $3.9\times 10^{13}~P$.
Work Step by Step
Let $P_M$ be the average power output of the magnetar.
Let $P$ be the average power output of the sun.
We can find the power output of magnetar.
$P_M\times (0.20~s) = P\times 250,000~years$
$P_M\times (0.20~s) = (P\times 250,000~years)(365\times 24\times 3600~s/year)$
$P_M = \frac{(P\times 250,000~years)(365\times 24\times 3600~s/year)}{0.20~s}$
$P_M = 3.9\times 10^{13}~P$
The average power output of this magnetar was $3.9\times 10^{13}~P$.
