Mark and his sister Chris both leave their house for school at the same time.  Mark travels at the same speed as Chris, who walks 3 miles per hour. After walking 3 miles Mark has to turn around and go home because he forgot his lunch. If the distance from their house to the school is 9 miles, how much longer does Mark spend walking than Chris?
First find the total number of miles Mark walks to get his lunch by multiplying the distance he walks back to the house by 2: 3 miles * 2 = <<3*2=6>>6 miles
Then find the total distance Mark walks by adding that distance to the distance between the house and the school: 6 miles + 9 miles = <<6+9=15>>15 miles
Now divide the total distance Mark walks by his speed to find how many hours he spends walking: 15 miles / 3 miles/hour = <<15/3=5>>5 hours
Now divide the total distance Chris walks by her speed to find the total time she spends walking: 9 miles / 3 miles/hour = <<9/3=3>>3 hours
Now subtract the time Chris spends walking from the time Mark spends walking to find the difference: 5 hours - 3 hours = <<5-3=2>>2
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