[ethbr1]

There's a difference in type of thinking in moving from operations on numbers (basic whole number math) to operations-on-operations-on-numbers (anything with fractions).

Suddenly, you need to begin to understand the rules around operators, sequencing, and what operations are legal and illegal.

Absent that understanding, even...

   1/4 x 2/5

... gets very complicated trying to reason with physical analogs.

So it's the point at which math becomes "pure" rather than strictly physically-mapped.

[Ekaros]

And if multiplying fractions is simple. Before that addition is done. Which is more complicated.

Actually I think we do very little addition of fractions later in math. But it is a concept that confuses the multiplication or division.