Mr. Montero is traveling a total distance of 600 miles + 600 miles = <<600+600=1200>>1200 miles.

There are 1200 miles / 400 miles = <<1200/400=3>>3 sets of 400 miles in 1200 miles.

So Mr. Montero needs 20 gallons/set x 3 sets = <<20*3=60>>60 gallons in all.

Since his car has 8 gallons already, then he needs to add 60 gallons - 8 gallons = <<60-8=52>>52 gallons more.

# Answer

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<|im_start|>system
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<|im_start|>user
A car uses 20 gallons of gas to travel 400 miles. Mr. Montero's car has 8 gallons in it. How many more gallons of gas does he need to travel 600 miles, back and forth?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
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user
52
A car uses 20 gallons of gas to travel 400 miles. Mr. Montero's car has 8 gallons in it. How many more gallons of gas does he need to travel 600 miles, back and forth?
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