Since the question is asking how many more hours 'till Ming would have 500 points, you subtract 200 from 500. That would result with the number 300. You get 20 points per hour. So 300 divided by 20 would be 15.
Ming needs to play for 15 hours to get 500 points. They gave him 200 for signing up, so 500-200=300 more points needed. 300 divided by 20= 15 more hours.
