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I don't know the probability of rolling a large straight with 5 six-sided dice, so I need to know what the probability is. What is it?
The probability of rolling 1-2-3-4-5 in that order is $(1/6)^5$. However any order will do, so it's $5!(1/6)^5$.
The probability of rolling 2-3-4-5-6 is similarly $5!(1/6)^5$.
Combining, we get $240(1/6)^5\approx 0.03$, i.e. about 3% of the time.