1. The problem statement, all variables and given/known data There are 17 street lamps along a straight street. In order to save electricity and not affect the regular use at the same time, we can shut down 5 of these lamps. But we cannot turn off a lamp at either end of the street, and we cannot turn off a lamp adjacent to a lamp that is already off. Under such conditions, in how many ways can we turn off 5 lamps? 2. Relevant equations 3. The attempt at a solution I've looked at this question a few times now and I still don't even know where to begin. Any help would be highly appreciated.