Questions on Interpreting Workout Instructions

I bought Workouts in a Binder the other day. I understand the terminology but on many of the workouts, the way they're written doesn't make sense to me and I'm hoping I might get some guidance on reading some of them. Here are a few: "W/U: 150 FR, 150 K, 150 FR, 150 PULL, 8 x 50 (25 IM, 25 FR) (:15RI)." How does one do a 25 IM? "W/U: 12 x 75 ." Is the "BK/BR/FR" a choice or am I suppose to change the stroke of the last length each interval? "4 x 75 BUILD and DESC (:20RI)." On intervals 2-4, do I start at a faster pace than I started on the previous interval or do I start at a faster pace than I ended on the previous interval? Or is the only thing that matters when comparing intervals is the descending time (i.e., the build applies only within the intervals and the descending time applies between the intervals)? Thanks! I'm sure with practice, I'll catch on to how to interpret these but I still need some help getting started.