Your calculation is incorrect. Marty walks to school 4 days each week, and each walk takes 15 minutes. To find out how much time it takes Marty to walk to school and back each week, you need to double the time it takes for one walk.
To solve this, you can multiply the time for one walk by 2, and then multiply by the number of days Marty walks to school in a week.
So, 15 minutes (time for one walk) × 2 (to account for the walk to school and back) × 4 (number of days Marty walks to school) = 120 minutes.
Therefore, it takes Marty 120 minutes, or 2 hours, to walk to school and back each week.