The length of time between customers (in minutes) at a certain register follows an exponential distribution with parameter 1/5. A cusomer just finished at the register. Let X=the amount of time until the next customer arrives at the register.
I believe the density function of X is: f(x)= (1/5)e^(-(1/5)x) for x =0 and the distribution function F(x)=1-e^(-(1/5)x) for x =0 - How do I figure out the average time between customers?

Question

The length of time between customers (in minutes) at a certain register follows an exponential distribution with parameter 1/5.  A cusomer just finished at the register.  Let X=the amount of time until the next customer arrives at the register.  
I believe the density function of X is: f(x)= (1/5)e^(-(1/5)x) for x >=0 and the distribution function F(x)=1-e^(-(1/5)x) for x >=0 - How do I figure out the average time between customers?