This one gave me problems, even after seeing the answer. I know it is too easy to get tricked by a flawed calculation, so like to apply a gut check. WHY is the answer different when the eruptions occur on average every 2, 4, and 6 hours versus occurring exactly every 2, 4, and 6 hours?
The best that I can come up with is that with even spacing, you are always relatively close to a time for A to erupt, so the likelihood of B or C erupting first is relatively low. With average spacing, there will be some instances when A's eruptions are close together, say 1/2 hour apart, and probability of seeing A first is greater if you get to the park between these two eruptions. However, you are much more likely to get to the park between two eruptions of A that are, e.g., 3 1/2 hours apart, when it is less likely that you will see another A before B or C.
But this is pretty soft reasoning -- if you can come up with a better explanation, please share!