CPhill, your answer is incorrect.
The probability that exactly 3 dice show the same is \({5 \choose 3} \times {1 \over 6^3} \times{5 \over6^2} \times 6\). \(5 \choose 3\) ways to pick the rolls that are the same, \({1 \over 6}^3\) chance of 3 successes, \({5 \over 6}^2\) chance of 2 failures, and 6 different trios of numbers that are the same.
The probability that exactly 4 dice show the same is \({5 \choose 4} \times {1 \over6}^4\times {5 \over6} \times 6\). With the same logic as above.
The probability that all 5 dice show the same is \({1 \over 6}^5\times6\).
Add everything up, and you get \(\color{brown}\boxed{23\over108}\)
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