If girlfriend remember any high school math, you"ll remind the following difficulty - in how numerous unique ways deserve to the letters of the word MISSISSIPPI be arranged? an alert there is repetition of some letters - I and also S each appear four times, when P appears twice.

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The Permutation formula

Since it"s an arrangement, bespeak matters, which is to say the MISSISSIPPI is a different arrangement from IMSSISSIPPI, acquired by switching only the an initial two letters. If there to be no repetition, us would usage the permutation formula symbolized by 11P11, and also find the end there are practically 40 million arrangements (39,916,800 to it is in exact). Since of the repetition, countless of those arrangements room the same, therefore we need to divide that an outcome by commodities of factorials for each that the repeating letters. (As a reminder, four factorial, symbolized through 4!, method four times three times two times one, which amounts to 24.) So, four factorial equals 24, and also there room two of those because that the letters I and also S. For the letter P, we use 2 factorial which equals two. So, we need to divide the vast number over by the product of 24 times 24 time 2: 






there is no the repetition, that course, there space enormously fewer arrangements. That"s all you"ll see in most high school math publications with regard come permutations v repetition.


What happens, however, if one of your bright college student asks the following question: How countless unique arrangements can be formed from the letters in words MISSISSIPPI if you want to form arrangements less than 11 letter long? because that example, how countless unique five-letter arrangements can be formed? This new problem is not hard, yet it will certainly be immeasurably beneficial to go ago to the initial problem and look in ~ it in a different way. Let"s perform that now, and also you"ll thank me because that it later.


The mix Formula 

In the original problem, we want to form arrangements using all of the letter in the word. Take into consideration that there room only 4 different species of letters in the word MISSISSIPPI - in order of decreasing frequency of appearance, they are I, S, P, and M. We have the right to now start the trouble by asking: How plenty of ways have the right to we species the 4 I"s in the 11 areas we must fill? since the four I"s space indistinguishable, us would use the combination formula stood for by 11C4, and get 330 ways. There are seven areas left to fill, so let"s move to the letter S and ask how plenty of ways can we kinds the 4 S"s in those seven spots - this would be 7C4, or 35 ways. There are three ways to species the 2 P"s in the three staying spots, i beg your pardon we obtain from 3C2, and also finally 1C1 provides us one means to put the M in the last continuing to be spot. The counting rule tells us to main point those 4 numbers together to obtain the total variety of ways those letters have the right to be arranged: 






(11C4)(7C4)(3C2)(1C1) = (330)(35)(3)(1) = 34,650.






notice that we have obtained the result we got earlier using a single permutation! it is worthwhile to keep in mind that the count principle provided us the unique variety of arrangements (permutations) ~ we offered combinations come take treatment of all the repetition. Really nice, don"t friend think? In specific situations, then, combine + the counting principle = permutations.


Now, ago to our bright student who has actually been waiting patiently for an answer. Equipped with what we now know, it is simple to prize his question. If we are creating five-letter arrangements, we begin again and also ask: How many ways deserve to the four I"s be arranged to fill the five places? This would be 5C4, giving 5 ways. We have just filled 4 of the five spots, leaving only one to it is in filled. There are three remaining varieties of letters, so we have the right to simply main point by three and we have actually our answer:






(5C4)(3) = (5)(3) = 15.

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Contact chathamtownfc.net for an ext Help through Discrete Math

Hopefully, this article will help students and teachers out there who discover themselves at the mercy of little combinatorial geniuses who work their means into your classrooms.


For more information, see A Discrete change To progressed Mathematics through Bettina and Thomas Richmond, released by the American math Society. Chapter 4 of the text is an extremely helpful on this topic. Lowell Parker, Ph.D. Realm State College