The average speed of a vehicle on a stretch of Route 121 between 6 a.m. and 9 a.m. of a typical weekday is approximated by the function, where f(t) is measured in miles per hour, and t is measured in hours, with t=0 corresponding to 6 a.m.
a) Find the interval where f is increasing and the interval where f is decreasing.
b) At what time of the morning commute is the traffic moving at the slowest rate? What is the average speed of the vehicle at that time?

**I already posted this question, and rated the very helpful response as a lifesaver. However, my teacher sent out an answer key that says the correct answer is as follows: the function is inreasing on (16,∞) and decreasing on (0,16) (this means the average speed is decreasing from 6am to 9am, since this corresponds to the interval [0,3]). The traffic is moving slowest at 9am. At that time, the average speed of a vehicle is approximately 35.79mph.

**The lifesaver who worked this problem the first time got the following answer: f is increasing on (1,3) (which is 6am to 9am) and decreasing on the interval (0,1) which is 6am to 7am. At t=1, or at 7am, traffic is moving the slowest at a speed of 24mph.

**If anyone could tell me how my teacher got this answer and where the 16 came from, I will rate lifesaver for certain. I am really confused about this problem. Thanks!