Download a free video editor called VirtualDub. (The DivX video codec is also nice, for letting you save compressed low-memory videos.)
With it, you can either trim a segment of the video out, or you can save the video as multiple parts, and have certain parts played in fast forward. For example, you'd save the 21 rep part in normal speed, and the 9 rep part at normal speed. The middle 15, being less exciting than the start or end, could be edited to play at a 1.5x framrate, then saved. Using Windows Movie Maker, the three clips could then be appended into one video, and have subtitles added, such as "this part in 1.5x fast forward."