Reach 100
Here is a grid of four "boxes":
You must choose four different digits from 1−9  and put one in each box. For example:
This gives four two-digit numbers:
| (reading along the | |
| (reading along the | |
| (reading down the left hand column) | |
| (reading down the right hand column) | 
In this case their sum is 
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 two-digit numbers which add to a total of 
How many ways can you find of doing it?
 
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