For a high-intensity, 50 minute class, do all 15 tracks.

 

For a high-intensity 40 minute class, skip tracks 12, 13, and 14.  

For a high-intensity,  30 minute class, skip tracks 10, 11, 12, 13, and 14

 

For a less intense, 40 minute class, skip tracks 4, 7, and 8. 

For a less intense, 30 minute class, skip tracks 4, 7, 8, 11, and 12.