You may also like

problem icon

Cubes

Investigate the number of faces you can see when you arrange three cubes in different ways.

problem icon

I'm Eight

Find a great variety of ways of asking questions which make 8.

problem icon

Let's Investigate Triangles

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?

Doing and Undoing

Stage: 1 Challenge Level: Challenge Level:1

 
Maybe you are used to making knots and sometimes you undo them!
 
Let's look at undoing some maths that's been done.
 
Suppose we have a starting number and then we doubled it.
That's the bit we'll call "doing".
To "undo" the maths we start with just the answer and see if we can get back to our starting number.
When I  doubled my starting number, I got $6$. What do I have to do, to "undo" the $6$ and get back to my starting number?
Suppose that I did it again with a new starting number so I doubled and got to $16$. What would you have to do to "undo" and get back to my new starting number?
 
Can you think what you would have to do to "undo" these three children's maths?
 
Danesh says
"I took $4$ away, what should I do to get back to my starting number?"
Meg says
" I added $8$, what should I do to get back to my starting number?"
Chris says
"I halved, what should I do to get back to my starting number?"

Now, if they all finished with a $12$ what were their starting numbers?
 

 

Photograph acknowledgements;

http://www.instructables.com/image/FPRA3T8FZ8J4A39/How-to-tie-various
-knots.jpg
http://2.bp.blogspot.com/-Jg0te1K_F5g/TpFcEY8xT6I/AAAAAAAAHy4/vgkke
-0QQ98/s1600/knot.jpg
http://www.instructables.com/image/F0PMDJ8FZ8J4A36/Figure-8-Knot.jpg