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

# Answer

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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?
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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?
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