what is 5f-f

when you are adding and subtracting 'letters' it is easier if you think of them as things, so

5f-f becomes 5 frogs-a frog = 5frogs -1 frog = 4 frogs = 4f

5f-f

If we factor out the f, we get f*(5-1) This then becomes f*(4) which is equivalent to 4f