[outop]

The average time between two buses (based on the Poisson model used in TFA) is N minutes. But you are more likely to arrive in a long interval than a short one. So if you turn up to the station at a random time, the average time between the last bus that departed before you got there and the next departure, is 2N minutes.

[stonemetal12]

Doesn't change the fact that N is 10 and evidently 5 in the same sentence. Does author think only 'N' can be used for variables and readers are left to figure out which 'N's are the same and which are different?

[outop]

No, N means 10 in both parts of the sentence you quoted.

> when the average span between arrivals is [10] minutes, the average span experienced by riders is [20] minutes.

>The average wait time is also close to 10 minutes, just as the waiting time paradox predicted.

The average wait time is half the 'average span experienced by riders', since the rider arrives, on average, halfway through the average span.