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Here is a grid of four boxes:




You must choose four different digits from 1  9 and put one in each box. For example:
5 
2 
1 
9 
This gives four twodigit numbers:
52 (reading along the first row)
19 (reading along the first row)
51 (reading down the lefthand column)
29 (reading down the righthand column)
In this case their sum is 151.
Try a few examples of your own.
Is there a quick way to tell if the total is going to be even or odd?
Your challenge is to find four different digits that give four twodigit numbers which add to a total of 100.
How many ways can you find of doing it?
You may be interested in the other problems in our Mastering Mathematics: Developing Generalising and Proof Feature.
This problem is adapted from Make 200 from 'Mathematical Challenges for Able Pupils Key Stages 1 and 2', published by DfES.