OK, a little clarification on cassette terminology is in order. Bill Kapun's writing of a 12/13-24/25 cassette means the smallest cog is either a 12 OR a 13 tooth and the largest cog is either a 24 OR a 25 tooth.

So a 12x25 cassette would have a 12T smallest and a 25T largest cog. The number and tooth count of the ones in between would depend on the total number of cogs in the cassette. In your case the total number would be 8 and a 12x25 cassette would most likely be made up as: 12,13,15,17,19,21,23,25