One of these things is not like the others
Please go to the Question #18 post if you haven't had a chance work on this one yet, since the text below contains the solution.
Whenever people see a list of numbers like this, the first instinct is to find a mathematical relationship between them. In this particular puzzle, that would lead you astray. It's actually a music question!
Three of these are record formats. The number is the speed (in RPMs) that you play the record. The "45" is probably the most familiar in this context, since that number at one point became synonymous with the fact that these records were "singles". Other records are played at 33 1/3 RPMs, and 78 RPMs. But there was never a record format played at 12 1/2 RPMs, so that is what does not belong.