A student has no idea where to place a fraction on a number line (because fractions aren't numbers, of course) but can convert to a mixed number like a champ.
My attempt to help out:
This applet gets at the heart of the things I've enjoyed working on lately. The initial estimate offers very little help, but as the student progresses through, they have more references which allow the revisions to become more precise. When my students worked with this applet, there were audible groans when I asked them to lower the lids on their computers as well as exclamations of "I got it!" when they moved closer to 0% error.
Here's a GeoGebra book that goes from estimating fraction to addition to multiplication. I'm still working on division, but that should drop soon.