anna will be F years old X years from now. how old is she now??
To find out how to do it, make up number for F and X. Let's make
up F=14, and make up X=5. If it were that way, the problem would be
anna will be 14 years old 5 years from now. how old is she now??
You can solve that easily and get that she must be 9 years old.
Now we ask ourselves this question: "How did we get 9?"
The answer to that question is: "We subtracted 14-5 and got 9."
So instead of subtracting 14-5 for our made-up problem, we do a
similar thing with our original problem and subtract F-X instead
of 14-5.
Answer: F-X
[Note, when we subtract two numbers, like 14-5, we get a third
number, like 9, but when we subtract two letters, like F-X, we
just indicate that subtraction F-X and consider that, just as
it is, "F-X", to be the third number.]
Edwin