In one of my online classes I discouraged students from using the same pair of primary sources in their essay that another student already used. I wanted to point out the possible number of combinations from a choice of 15 sources, but I could not remember the formula for calculating permutations.
Hey - it has been 26 years since I last attended a math course of any kind, and frankly mathematics began to stop making sense to me at the calculus level.
My first effort was to query my math-genius wife, who has an engineering degree and a Master's in mathematics curriculum and instruction, and who is working on a second Master's in mathematics. Unfortunately, she always uses her calculator, which is programmed for such high-falutin' mathemagicery.
I next went to Google, and after a few searches I found a permutations calculator that computes the combinations for you after you enter the relevant data (number of items, numbers of choices, whether order matters, and if repetition is permitted). Voila! The answer was 105 combinations, meaning that my 32 students should have no difficulty whatsoever avoiding duplication of efforts.
However, at least six students will ignore my detailed instructions and post duplicate material anyways that bears a suspicious similarity to the work of earlier respondents (this is an online course where students read and respond to the essays of other classmates). Yes, there will always be people who cannot follow clear instructions, even something as clear as a link that says to click here to learn more about Quick Trim.
Oh, and the formula is P(n, r) = n!/(n-r)! , where n is the number of elements available for selection, and r is the number of elements to be selected (0 ≤ r ≤ n).
In case you were wondering, Bubba.