Calculating Average Wait Time for a McDonald's Drive-Through Window

[tachu101]

Homework Statement

Queuing Theory (study of lines for stores) says that for a drive through window at a Macdonalds, the function

f(x)= 9/(x(x-9))

represents the average time in hours a customer will wait in line. X=average number of people an hour.
How long will a customer have to wait in line (on average)?

The Attempt at a Solution

Not sure at all on how to find an average time out of this function.

 

[Kurdt]

Were you given a value for x?

 

[tachu101]

The question I think says 9<x<or=20

 

[HallsofIvy]

Yes, x will have to be larger than 9 for that function to be positive- and I've never had a negative waiting time at McDonald's! The average of a function, f(x), over $a\le x\le b$ is
$$\frac{\int_a^b f(x)dx}{b-a}$$