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On dividing a number by 5, we get 3 as remainder. What will the remainder when the square of the this number is divided by 5?

Correct Answer: 4

Explanation:

Let the number be x and on dividing x by 5, we get k as quotient and 3 as remainder.

∴    x = 5k + 3


⟹    x2 = (5k + 3)2


   = (25k2 + 30k + 9)


   = 5(5k2 + 6k + 1) + 4


∴On dividing x2 by 5, we get 4 as remainder.


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