First, we determine that Isabella will be 10*12= <<10*12=120>>120 months old at age 10, to find a number to compare her age to Antonio's with.

Then we subtract the 18 months from the total of 120 months to find 120-18= <<120-18=102>>102 months, which is the age Isabella is currently.

Since at 102 months old Isabella is twice as old as Antonio, we take 102/2= <<102/2=51>>51 months as Antonio's age.

# Answer

51
<|im_start|>system
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<|im_start|>user
Isabella is twice as old as Antonio.  In 18 months, she will be 10 years old.  How many months old is Antonio?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
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user
51
Isabella is twice as old as Antonio.  In 18 months, she will be 10 years old.  How many months old is Antonio?
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