So what I gather from your code, is this:
Set up a 3x3 Matrix
Wait until user presses a keycode above 70
If keycode is in the interval (70,80], B is 8
If keycode is in the interval (80,90], B is 6
If keycode is in the interval (90,105], B is 4
You take the iPart of the keycode, which will be in the set {7,8,9, 10}, subtract C, and add 10fPart of the keycode which will be in the set {1,2,3,4,5}
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This is where it gets tricky. What we know now is that A will be one of the numbers in the set {8,9,10,11,12,13,14,15}, minus C. It is important to note that certain keys such as 72 and 81 will result in the same A although they have different B's. It would be helpful to know what C is, since I don't see it being set anywhere in the code.
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"If int(A/3)" will always be true, as long as A is not 0 or 2 so A equals A-1.
At this point, A is one of {7,8,9,10,11,12,13,14} minus C
If you take int(A/3)+1, and store it to X, it has to be in the set {1,2,3}, so int(A/3) has to be in the set {0,1,2}. Which means A has to be between 0 and 8. Because of this, C has to be either 6 or 7
3fpart(A/3) also has to be in the set {1,2,3}. This means that A cannot be divisible by 3, since it will result in 0 if it is. 3fpart(A/3) can result in {0,1,2}, so you really should be using 1+3fpart(A/3)
I probably didn't explain all that the best I could have, but basically the lines where you set X and Y are dependent upon what A and C are. You should change $3fpart(A/3$ into $1+3fpart(A/3$ because if A is divisible by three it will result in 0. If C is anything other than 6 or 7, your code won't work either. I think you may need to double check your if statements or make sure you're using int( in the right way. If you need more help, posting the full code as well as the purpose of the code would be the easiest way
