Answer:
"=RAND()*(40) + 20"
Explanation:
When we say that a random variable X follows an uniform distribution
where a is the lower limit and the upper limit we are saying that each value on the interval (a,b) have the same probability of occur, and the density function is given by:

And
for any other case.
For this special case we know that the random variable T="Waiting times at a toll booth" follows this distibution:

And we can use the excel to create random values on this range. So then the code to do this is given by:
"=RAND()*(b-a) + a"
"=RAND()*(60-20) + 20"
"=RAND()*(40) + 20"