In a bus station the probability of a bus coming in the next minute is $1/10$.
If we define $X$ as a random variable who point at which minute the bus is coming, it's easy to see that $X$ is geometrically distributed with $p = 1/10$.
So we know that $E(X) = 10$.
Now say we got to the bus station on a random time, what is the expected value between the last bus that arrived and the next bus ( the one we are waiting for).
I'm a bit confused as on one hand it's seems it should be $10$, but on the other hand it's seems like it should be $20$...