Thrusters, Chest to bar Pull-ups

Complete as many reps as possible in 7 minutes of the following rep scheme:

100 pound Thruster, 3 reps
3 Chest to bar Pull-ups
100 pound Thruster, 6 reps
6 Chest to bar Pull-ups
100 pound Thruster, 9 reps
9 Chest to bar Pull-ups
100 pound Thruster, 12 reps
12 Chest to bar Pull-ups
100 pound Thruster, 15 reps
15 Chest to bar Pull-ups
100 pound Thruster, 18 reps
18 Chest to bar Pull-ups
100 pound Thruster, 21 reps
21 Chest to bar Pull-ups...
This is a timed workout. If you complete the round of 21, go on to 24. If you complete 24, go on to 27, etc.

I finished the round of 15 reps with a few seconds to spare.  I would like to get this far pulse 18 Thrusters next time.  I just did not know how to play this one.  Good quick WOD.