You can't compare two fractions on two number lines unless they are exactly the same scale and perfectly partitioned.
Try using a number line to determine which is greater 5/9 or 9/16.
That's really the point; you can use a number line to demonstrate the process, but mandating that the kids use the number line to solve the problem will only confuse them.
I demonstrated in post#25 they need not be the same scale.
Perfectly partitioned, yes.
I doubt solutions using geometric construction is typical 5th grade curriculum.
Yes, after I saw the “triangulation” method diagram someone posted, I understand what they’re getting at. But, even though that method could work with number lines that are out of scale, you’d still need the individual segments of each line to be equidistant, which is another thing you are never supposed to assume with number lines.
Basically, by teaching this method, they probably figure they are making it easier for kids than the old “common denominator” method, but they are actually teaching kids to disregard a couple basic principles of mathematics.