Eustace’s current age must be 39 years old – 3 years = <<39-3=36>>36 years old.

So Milford’s current age must be 36 years old / 2 = <<36/2=18>>18 years old.

So in 3 years, Milford will be 18 years old + 3 years = <<18+3=21>>21 years old.

# Answer

21
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Eustace is twice as old as Milford. In 3 years, he will be 39. How old will Milford be?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
21
Eustace is twice as old as Milford. In 3 years, he will be 39. How old will Milford be?
train