How do you simplify #9/10 - 4/5#?
Let's start with the original:
When we deal with fractions, we want to make sure we are dealing with same terms. Or if we want to talk about the size of pizza slices, it's the denominator that tells us how big the slices of pizza are and it's the numerator that tells us the number of those slices (a denominator of 10 says it takes 10 slices to make a whole pizza whereas a denominator of 5 tells us it takes 5 slices to make that same size pizza).
Right now we are dealing with 2 different sizes of pizza slices and we really want to work with one size. And we can do that most easily by expressing the "4 slices of a 5 slice pizza" and expressing it as a 10 slice pizza, like this: