2009 UNCO Math Contest II Problems/Problem 2

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Problem

(a) Let $Q_n=1^n+2^n$. For how many $n$ between $1$ and $100$ inclusive is $Q_n$ a multiple of $5$?

(b) For how many $n$ between $1$ and $100$ inclusive is $R_n=1^n+2^n+3^n+4^n$ a multiple of 5?


Solution

(a) Looking at the units digits, we need the units digit of $Q_n$ to be either $0$ or $5$. We know that $1^n$ will always have a units digit of $1$. Looking at $2^n$, however, cycles every four powers with units digits $2, 4, 8,$ and $6$ in that order. We see that we can only get a units digit of $5$ if we have $4$ as a units digit for $2^n$, and there is no way to get $0$ as a units digit. Therefore, our answer is $\boxed{25}$ because the four units digits cycle $25$ times in the integers $1$ to $100$.

See also

2009 UNCO Math Contest II (ProblemsAnswer KeyResources)
Preceded by
Problem 1
Followed by
Problem 3
1 2 3 4 5 6 7 8 9 10
All UNCO Math Contest Problems and Solutions
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