Since she seems to be burning a constant (same) amount of calories per minute we can find the answer with simple division. So in 45 minutes she burned 225 calories, think about how much she would burn in 1 minute. To find the answer you would divide 225/45 to get 5. If you want to check this answer you could multiply 5 by the number of minutes she hiked and see if it matches up with the table. (ex. 90*5=450).
