#### Answer

255 sec.

#### Work Step by Step

$Q_{3}$ is the value where $\frac{3}{4}=75$ % of the values are smaller than $Q_{3}$. 75% of the data here is: $24\cdot0.75=18$, therefore $Q_{3}$ is the average of the 18th and 19th value, which is (255+255)/2=255 sec..