Primal Problem Generator

This applet allows you to explore computation with numbers that have 2,3,5 and 7 as factors. Can this applet generate problems that cannot be solved in the applet? What is true about such problems? How many ways can you solve a problem that can be solved? What questions could/would you put to your students based on this applet? [N.B. In 'easy' mode results the desired results of the computation lie between 1 and 10. In 'hard' mode the desired results of the computation lie between 1 and 100]