Answer: M - 21
Step-by-step explanation:
Since his daughter is 21 years younger than he is (M) all you have to do is take his age and subtract 21 (how many years older he is) to get her age.
At first this might seem difficult because you don't know what M is equal to but in the context of this question you don't need to know M.
If you're ever stuck just give an imaginary number to M such as 50.
If you take 50 and subtract 21 you get 29, this makes sense because his daughter is of course younger than him.
You can also use this tatic to solve problems like x+3=x/2 (or something like that) just to check and make sure that the two equations do equal each other.
:)
