I am working through Israel Gelfand's book "Algebra" and am stuck on the following problem:

Find all possible ways to put parentheses in the product 2*3*4*5*6 (not changing the order of factors). Try to invent a systematic way of searching so as not to forget any possibilities.

I calculated 16 possibilities by rote before giving up (there may be more), and the only thing I accomplished in the way of "systematic" was stating that in any sequence of five numbers there are four operations to be performed. Perhaps it's the late hour, but I can't get beyond this. Any suggestions?

Thanks!