How can we visualize the process of adding or subtracting fractions with unlike denominators? The Web Sketchpad model below (and here)  offers tools for representing fraction addition and subtraction on a number line as well as a parameter (called “divisions”) that allows you to find a like denominator through visual inspection rather than calculation.

And below (and here)  is another model for adding two fractions, each less than one. Type the numerator and denominator of the two fractions directly into the boxes and change the number of divisions by dragging the slider point to the right.

You can watch a short video of both models in action here.