Answer:
Week 1: 10 Minutes
Week 10: 77.5 Minutes or 1 hour and 17.5 Minutes
Week n: 10 + 7.5n
Explanation:
Every week, Greg adds another 7.5 minutes to his workout time. Week 1 starts off with 10 minutes and continuously adds 7.5 minutes until he gets to week three, which 10 + 7.5(2) = 25 minutes.