Fraction Workbench - decomposing fractions

Topic:
Fractions
The easy challenge in this applet asks the user to decompose a fraction into the sum or difference of two fractions that have the same denominator. A harder challenge is to decompose a fraction into the sum or difference of two fractions with different denominators. Harder still is the challenge to decompose a fraction into two fractions neither of whose denominators is the same as that of the original fraction. What questions could/would you put to your students based on this applet?