JEE Advanced Mathematics question

If the integers and are chosen at random between 1 and 100 then the probability that a number of the form is divisible by 5 is

Answer: (C)

Solution

The units digit of depends on :

| | 1 | 2 | 3 | 0 | |---|---|---|---|---| | units digit of | 7 | 9 | 3 | 1 |

Among each residue class mod 4 has exactly 25 numbers. So for independently chosen the pair is uniform over the 16 combinations,

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Question ID VK-109759