The courier can drive anywhere between 50 mph and 65 mph. Which speed will give the shortest time? Which speed will give the longest time?

Plug $r = 65$ mph and $r = 50$ mph into $t = \frac{d}{r}$ with $d = 300$. Then think: should $t$ be between those two results, and which one should be the smaller (left) endpoint of the inequality?

In one expression line, type 300/65 and note the decimal value that Desmos gives; this is the time it takes to go 300 miles at 65 mph (the shortest possible time).

In a second expression line, type 300/50 and note the decimal value; this is the time it takes to go 300 miles at 50 mph (the longest possible time).

Compare the two times you found: one is smaller, one is larger. All possible values of $t$ are the numbers between these two times, including both endpoints. On your paper, write an inequality for $t$ with the smaller time on the left and the larger time on the right.

The basic relationship is

For this problem, the distance is fixed at $300$ miles, the rate (speed) is between $50$ and $65$ mph, and $t$ is the time in hours.

Starting from $d = rt$, solve for $t$:

Here $d = 300$, so

This means the time depends on which allowed speed $r$ (between 50 and 65 mph) the courier chooses.

To get the shortest time, use the highest speed, $r = 65$ mph:

To get the longest time, use the lowest speed, $r = 50$ mph:

Because $\frac{60}{13}$ is less than $6$, the time $t$ must satisfy

So the correct description of all possible values of $t$ is $60/13 \le t \le 6$.