When looking on Resourceaholic, I found this lesson by David Gale taken from the TES. I don’t usually take Powerpoints from the TES, but thought this one was pretty good and up to date.  The first slide also uses the Craig Barton method of Goal Free Problems that we looked at during one of our MAT sessions.

It has a couple of minor changes from the orignal

Objective	Understand and use Frequency Trees
Resources	Powerpoint