Mel is three years younger than Katherine. When Katherine is two dozen years old, how old will Mel be in years? Let's think first. Chain of thought:
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.
Therefore, the answer is 21.