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'What Is Ziffle?' printed from http://nrich.maths.org/
Why do this problem?
This short problem
encourages children to look for patterns and apply knowledge of their times tables and of the properties of numbers.
Encourage the children to look at the sets of numbers in turn and explore their characteristics and properties.
Are they all odd in one set and even in the other?
Are they prime numbers?
Are they triangle numbers?
Are they all in the five times table?
How about other tables?
Either you or your pupils could make up their own problem using, for example, multiples of $13$ or another table that they might like to practice.
Take an easier table and develop sets of multiples and non multiples to identify.
Use concrete apparatus to explore divisibility such as Multilink cubes.