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'Exploring Number Patterns You Make' printed from http://nrich.maths.org/
If you're a teacher read the notes here first.
Have a go at becoming a detective!
Start by watching this video:
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You can now explore this further. Click on the picture to get started.
When you've explored what you can do with $3 \times 4 - 5$ then it's time to explore further.
You could change just one part of the number plumber, for example the $-5$ bit.
You might try $3 \times 4 - 6$ or $3 \times 4 + 5$ or $3 \times 3 - 5$ and compare the results.
You'll have lots of your own ideas about things to explore too.
Mathematicians like to ask themselves questions about what they notice.
What possible questions could you ask?
These questions may lead you to come to some decisions about what can happen.