Answer: 60mph
Explanation:
For each piece of our piecewise function, we have a different range for t, so we only use the expression that has a range that our t falls into. In the first, the range is 0 ≤ t ≤ 15, so the first expression ONLY WORKS for values of t that fit in this range (13, for example). In the third, the range is 240 ≤ t ≤ 250, so the third expression ONLY WORKS for values of t from 240 to 250 (242, for example). Our t does not fit in these ranges, so we do not use the first or third expression to find our answer.
In the second, the range is 15 < t < 240, so the second expression ONLY WORKS for values that are between 15 and 240, and our t is 18, so we use the second expression to find our answer. Our expression is just 60, so our final answer is at 18 seconds, the vehicle is going 60mph.
I hope this helps! :)