On CD8 there's also a version from 13 to 32, and to be honest I usually prefer
that one from the beginning, but see how you go with your students.
On the CD there are also two computer games, one is a straight quiz of numbers 13
to 20, but the other is a listening quiz to distinguish between 13 and
30, 14 and 40 and so on. This is often something that needs to be practiced.