Here are some resources to help familiarise your students with the preliminary materials
AQA
Guess the weight
Guess the weight
One jar 221g
5 jars 996g
249g
Some exam style questions with marking guide.
Please use with caution. Ppt format so you can adapt if you wish.
Some questions and answers
Some questions and answers
I've attempted to redraw the diagram and have some questions about it.