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Adam and Dylan had some great ideas, Sam explains his thoughts clearly:
Using $3z$ and $4z$ the highest number that can't be made is 5. This is because you can make 6, 7 and 8.
Mrs Dillon's year 7 class had a different way of showing 7 is the largest amount that can't be made;
We think using a $3z$ & $5z$ coin you can make all numbers except: 1, 2, 4, 7Fantastic!
Tze Liang Chee looked into other possible combinations for other countries:
An investigation involving adding and subtracting sets of consecutive numbers. Lots to find out, lots to explore.