A 150-pound person burns 5.4 calories per minute when walking at a speed of 4 miles per hour. While walking, this person eats a snack that has 90 calories. This snack subtracts from the calories burned while walking. How long must the person walk at this speed to burn at least 170 calories? Use pencil and paper. Describe what values the person could change so that the amount of time spent walking would be less.