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Pat has 17 sweets in her bag. Rebecca found the next solution by taking groups of five sweets away at a time - number 12 doesn't work. Why doesn't 12 work?

Alistair from Histon and Impington Infants School said:

We know that the number of sweets is odd, because she has one left over when she counts in twos but 7 does. She could have 7 sweets or she could have 17 sweets, but she can't have 12 because it is an even number. There are no other numbers under 20 that fit these rules.


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Andrew decorated 20 biscuits to take to a party. He lined them up and put icing on every second biscuit and different decorations on other biscuits. How many biscuits weren't decorated?

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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