700 Jumping Jacks
1) Gathering the data:
Training Time: 5:00 - 6:30 = 1:30 (one hour and a half)
10 Jumping Jack per minute.
One 20 minute break
2) Since there was only one 20 minute break then Arnold exercised for 1:30 hour - 20 min = 1 hour and 10 minutes
1 hour and 10 minutes are equivalent to 70 minutes for 1 hour = 60 minutes.
10 Jumping Jacks per minute in 70 minutes yields:

3) Hence, Arnold did 700 Jumping Jacks