When Katherine is 2 dozen years old, she will be 2*12=<<2*12=24>>24 years old.

If Mel is three years younger than Katherine, then when Katherine is 24 years old, Mel will be 24-3=<<24-3=21>>21 years old.

# Answer

21
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Mel is three years younger than Katherine.  When Katherine is two dozen years old, how old will Mel be in years?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
21
Mel is three years younger than Katherine.  When Katherine is two dozen years old, how old will Mel be in years?
train