ASM 27.2 and 27.3

[Just In Time]

Would anyone be able to help me understand the difference between ASM 27.2 and 27.3 (the former at least is a previous exam problem)? In 27.2 we only calculate v and in 27.3 we calculate v + a.

I'm not sure why these are treated differently, since I remember learning for M that if you have a Poisson process with mean lambda and possible event classes i, j, k with probabilities p_i, p_j, p_k of occuring (given an event occurs), then you can also treat them as three independent Poisson processes with means lambda * p_i, etc. Is the reason we can't do that in problem 27.3 just because it it is not a Poisson process but a -compound- Poisson process?