For time:
Squat 5 intervals of 20 seconds on 10 off,
Run 400 meters,
Squat 4 intervals of 20 seconds on 10 off,
Run 400 meters,
Squat 3 intervals of 20 seconds on 10 off,
Run 400 meters,
Squat 2 intervals of 20 seconds on 10 off,
Run 400 meters,
Squat 1 interval of 20 seconds.
Consider variations of the above workout that would keep the total volume constant (1600 meters and 15 squat intervals) and their likely impact on your time and reps.
If this workout were denoted 5-400-4-400-3-400-2-400-1 other workouts of equal volume might include: 5-800-5-800-5 and 800-15-800 to name two of a myriad of possibilities.