Mel is three years younger than Katherine.  When Katherine is two dozen years old, how old will Mel be in years?
How old will Katherine be when she is 24 years old? ** When Katherine is 2 dozen years old, she will be 2*12=<<2*12=24>>24 years old.
How old will Mel be when Katherine is 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.
#### 21