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Official Solution:




The unit's digit of 7 in a positive integer power repeats in blocks of 4: 7-9-3-1. Since \(75=4*18+3\) then the unit's digit of \(7^{75}\) is the same as the unit's digit of \(7^3\), which is 3.

Therefore, the unit's digit of \(7^{75} + 6\) will be: 3 plus 6 = 9.


Answer: E

I don't understand that. I assumed that 75 is a multiple of 5, and 7^5 ends in 7, so I 7 + 6 = unit digit of 3. Where does that 4*18 + 3 come from? I mean.. I could simply say 75 = 9*8 + 3? Is that how you decide what unit digit it is?

1. 7^1=7 (units digit is 7)
2. 7^2=9 (units digit is 9)
3. 7^3=3 (units digit is 3)
4. 7^4=1 (units digit is 1)
5. 7^5=7 (units digit is 7 again!)
...

So, the units digit of 7^(positive integer) repeats in blocks of 4: {7, 9, 3, 1} - {7, 9, 3, 1} - ... (cyclicity of 7 in power is 4).

Now divide 75 (power) by 4 (cyclisity): the remainder is 3. So, the units digit of 7^75 is the same as that of the units digit of 7^3, which is 3.


Check Units digits, exponents, remainders problems directory in our Special Questions Directory.
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I've revised the question and solution, incorporating additional details for improved clarity. I trust this makes it more comprehensible.
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