Answer
247.5 sec.
Work Step by Step
$P_{50}$ is the value where 50 % of the values are smaller than $P_{50}$. 50% of the data here is: $24\cdot0.5=12$, therefore $P_{50}$ is the average of the 12th and the 13th value, which is (245+250)/2=247.5 sec..
