2520 bolts
Explained:
If it starts working at 3 PM, and makes 900 nuts at the rate of 600 nuts per hour. 900/600 =1.5. So it stopped making nuts at 4:30.
Then it started making bolts at a rate of 720 per hour for 3.5 hours between 4:30 and 8:00pm. 720 x 3.5 = 2520 bolts