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I agree with guest EXCEPT   1  will divide    1,2   1,3  1,4  1,5  

so   5 choices out of 10    5/10 = 1/2 

Since 5C2 = 10 combinations of 2 numbers as follows:

{1, 2} | {1, 3} | {1, 4} | {1, 5} | {2, 3} | {2, 4} | {2, 5} | {3, 4} | {3, 5} | {4, 5} (total: 10)

Therefore there is only 1 such combination {2, 4} out of a total of 10.

Probability = 1/10