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### Number and algebra

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# Doing and Undoing

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Age 5 to 11

Challenge Level

I wonder if you've ever used string or rope to make a knot, and then undone it afterwards. In this activity, we're going to look at undoing some maths that's been done.

To begin with, we're going to imagine taking a starting number and then doubling it. We want to undo the doubling by starting with just the answer and trying to get back to our starting number.

When I doubled my starting number, I got 6. How can I undo the doubling and get back to my starting number?

When I doubled my starting number, I got 6. How can I undo the doubling and get back to my starting number?

Suppose that I did this again with a new starting number, and when I doubled it I got 16. How can I undo the doubling?

How would you undo these three children's maths?

Danesh says:

"I took 4 away. How can I get back to my starting number?"

Meg says:

"I added 8. How can I get back to my starting number?"

Chris says:

"I halved. How can I get back to my starting number?"

If these children all ended up with the number 12, what were their starting numbers?

Vincent and Tara are making triangles with the class construction set. They have a pile of strips of different lengths. How many different triangles can they make?