Using the uniform distribution, we have that:
a) The density curve is given at the end of this question.
b) 16.67% of the time does a customer have to wait between 100 and 120 seconds.
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The uniform distribution has two bounds, a and b, and the probability of finding a value between c and d is given by:

- Uniform distribution on the interval of 0 to 120 seconds, thus
.
The proportion between 100 and 120 seconds is:

0.1667*100% = 16.67%
16.67% of the time does a customer have to wait between 100 and 120 seconds.
A similar problem is given at brainly.com/question/15855314