Why fractions break algebra later
Most students who struggle with algebraic fractions in Year 10 are really still struggling with fractions from Year 7.
Ask a class of Year 10s to simplify an algebraic fraction and a good number will add the denominators. They are not being careless. They are doing exactly what they have quietly believed since Year 7.
See it break
Drag both sliders to 2, so the sum is a half plus a half. What does adding the bottoms claim the answer is?
A fraction is a count of slices, and you can only count slices that are the same size. Adding the bottoms changes the slice size mid-sum, which is why the answer comes out too small.
This is why we test fraction arithmetic before we teach algebraic fractions. If the idea underneath is shaky, the algebra has nothing to stand on.