###Human: Mel is three years younger than Katherine.  When Katherine is two dozen years old, how old will Mel be in years?, ### Assistant: When Katherine is 2 dozen years old, she will be 2*12=24 years old.
If Mel is three years younger than Katherine, then when Katherine is 24 years old, Mel will be 24-3=21 years old. 
 So the answer is :  21