Answer:
The number of minutes advertisement should use is found.
x ≅ 12 mins
Explanation:
(MISSING PART OF THE QUESTION: AVERAGE WAITING TIME = 2.5 MINUTES)
Step 1
For such problems, we can use probability density function, in which probability is found out by taking integral of a function across an interval.
Probability Density Function is given by:

Consider the second function:

Where Average waiting time = μ = 2.5
The function f(t) becomes

Step 2
The manager wants to give free hamburgers to only 1% of her costumers, which means that probability of a costumer getting a free hamburger is 0.01
The probability that a costumer has to wait for more than x minutes is:

which is equal to 0.01
Step 3
Solve the equation for x

Take natural log on both sides

Results
The costumer has to wait x = 11.53 mins ≅ 12 mins to get a free hamburger