Yes, they form a proportion. If we divide Nora's 70 pushups by the 2 minutes it took her, we get 35 pushups per minute. If we divide Luke's 105 pushups by the 3 minutes it took him, we again get 35. The difference between their number of pushups is 35, and the difference in their times is 1 minute. This shows both of them were doing 35 pushups per minute, making the rates proportional.