Question 1192796
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I suppose you want (X - Y) mod R in the form  A + RN where 0 ≤ A < R,
and where N is any integer.

(48 - 15) mod 10<pre>
                  
48 - 15 = 38, it's greater than 10, subtract 10, get 28. Still
greater than 10.  Subtract 10, get 18. Still greater than 10,
subtract 10, get 8.
      
Answer: 8 + 10N

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(24 - 31) mod 5<pre>
                    
24 - 31 = -7, it's negative, so add 5, get -2. Still negative, add 5,
get 3   

Answer: 3 + 5N 

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(62 - 85) mod 12

62 - 85 = -23, it's negative. so add 12, get -12. Still negative,
add 12 again, get 1.  

Answer: 1 + 12N

Edwin</pre>