Answer
It would not be unusual to observe 75 on-time flights from Dallas to Chicago.
Work Step by Step
If the observation is at least 2 standard deviations away from the mean (less than $\mu - 2\sigma$ or greater than $\mu + 2\sigma$) then it is considered unusual.
$\mu - 2\sigma$ = 80 - 2(4) = 72
$\mu + 2\sigma$ = 80 + 2(4) = 88
Since 75 falls between 72 and 88 it is not an unusual value.
