A number theory problem by Shubham Haldēr
Number Theory
Level
pending
If n is a positive integer such that the units (ones) digit of n2+4n is 7 and the units digit of n is not 7, what is the units digit of n + 3?
The answer is 2.
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1 solution
By plugging in values of n, we find that for n = 9,
n2 + 4n = 92 + 4•9 = 81 + 36 = 117.
So n = 9 works, and n + 3 = 9 + 3 = 12. So the units digit of n + 3 is 2.