His new car costs 1800+2000=<<1800+2000=3800>>3800 dollars.

His old car cost 3800/2=<<3800/2=1900>>1900 dollars.

# Answer

1900
<|im_start|>system
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<|im_start|>user
Ben's new car cost twice as much as his old car. He sold his old car for $1800 and used that to pay off some of the cost of his new car. He still owes another $2000 on his new car. How much did his old car cost, in dollars?
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<|im_start|>assistant

user
1900
Ben's new car cost twice as much as his old car. He sold his old car for $1800 and used that to pay off some of the cost of his new car. He still owes another $2000 on his new car. How much did his old car cost, in dollars?
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