I came across the following solved question in a book and am having difficulty understanding the answer given for it.
Q: A conference is to have eight presentations over the course of one day, consisting of three long presentations, and five short presentations. If the conference organizer doesn't want consecutive long presentations, and the conference is to start with a short presentation, how many schedules of presentations are possible?
A: Let S and L stand for a short and long presentation respectively. The schedule must start with a short presentation, and every long presentation must be followed by at least one short presentation. So the schedule must be SLSLSLS, along with one more short presentation inserted somewhere. There are four ways to do this - SSLSLSLS, SLSSLSLS, SLSLSSLS, and SLSLSLSS. For each of these there are 5! * 3! possible schedules, giving a total of 2880 schedules.
My analysis gave 7200 schedules instead as follows.
The first S is fixed. This leaves four S's and three L's to be permuted with the no-two-consecutive-L restriction. Or let's generalize it to a S's and b L's. Then the number of compatible permutations, written as N(a, b), can be expressed as the recurrence N(a - 1, b) + N(a - 1, b - 1). Observing the terminal values N(0, 1) = 1, N(1, 0) = 1, N(1, 1) = 2 and N(0, 2) = 0, and substituting a = 4 and b = 3, the number comes to 10. This gives a total of 10 * 5! * 3! = 7200 schedules.
What am I missing?