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Double Digit

Choose two digits and arrange them to make two double-digit numbers. Now add your double-digit numbers. Now add your single digit numbers. Divide your double-digit answer by your single-digit answer. Try lots of examples. What happens? Can you explain it?

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Reverse to Order

Take any two digit number, for example 58. What do you have to do to reverse the order of the digits? Can you find a rule for reversing the order of digits for any two digit number?

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Repeaters

Choose any 3 digits and make a 6 digit number by repeating the 3 digits in the same order (e.g. 594594). Explain why whatever digits you choose the number will always be divisible by 7, 11 and 13.

Always a Multiple?

Stage: 3 Challenge Level: Challenge Level:2 Challenge Level:2

Watch the video to see Charlie's number trick.

 

If you can't play the video, you can read a description here.

Try a few examples for yourself. Do you always get a multiple of 11?

Can you explain why?

 

 Alison and Charlie came up with their own explanations:
 


 


If you can't play the videos, you can read a description here.

 

Here are some similar number tricks.
Can you use Charlie's or Alison's representation to explain how they work?
 

  • Take any two-digit number. Reverse the digits, and subtract your answer from your original number. What do you notice?
     
  • Take any two-digit number. Add its digits, and subtract your answer from your original number. What do you notice?
     
  • Take any three-digit number. Reverse the digits, and subtract your answer from your original number. What do you notice?
     
  • Take any five-digit number. Reverse the digits, and subtract your answer from your original number. What do you notice?

 
Once you've been able to explain what is going on above you should be able to explain why many other similar tricks work.

Here is a selection you might like to try:

Special Numbers
Think of Two Numbers
Legs Eleven