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# Permutations & Combinations ?4C2 ? 11C5?

## Permutations and combinations homework help best texts!

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Although the other 6 questions don't ask for it, I ask them to try and write each question using similar notation.

If they are unable to find a way, I ask them to skip it and review it with us during the summary at the end. Perhaps the funnest aspect of a permutation is its connection to codes and code breaking.

Question 5 taps into that context and because of that I always review it with the class. One misconception that students face is how to set up a problem that has digits and letters. To get them started, it is important to think of the permutations of digits and then the permutations of letters and then multiply them. With the digits, students often misinterpret as nine available digits. We always step back and ask, "why is that wrong? Letters are equally confusing for me.

I tell them that I feel a bit nervous whenever a question uses the alphabet, since I can never remember how many letters are in the alphabet. Fortunately they tell us that there are 26 letters in the alphabet phew! Here we discuss the significance of not allowing repetition. In the second part of the question, we restrict the password by not allowing repetition of digits. I ask students, before we calculate, "why does it make sense that this would reduce the number of possible combinations?

Sign Up Log In. Unit 10 Unit 1: Starting Right Unit 2: Scale of the Universe: Making Sense of Numbers Unit 3: Fluency and Applications Unit 4: A Permutation is a set of objects is a listing of the objects in some specific order.

The number of different permutations of n is given by n! It is one of the different arrangements of a group of objects where order matters. The number of permutations of n objects taken r at a time is: Find the number of ways to arrange 4 people in groups of 3 at a time where order matters. Therefore, there are 24 ways to arrange 4 items taken 3 at a time when order matters. A Combination on the other hand is a mix of different objects, with no order.

A Combination is one of the different arrangements of a group of items where order does not matter. The number of combinations of a group of n objects taken r at a time is: I didn't notice that. How would I calculate the number of 2-seat segments for other problems in the future?

Just list them out? No you can think about where to put the first person and know that the second person will have to sit right next to them. So it will always be n- x-1 where n is the total number of seats and x is the number of people sitting together. I don't get why that formula works. I can see that it yields the numbers we're looking for but I'm not convinced of its workings.

I'm looking at it like this. You have 12 seats, let's say we fix Mary to be in the 1st, so Frances will have to sit in the 2nd. For the next arrangement, we fix Mary to be in the 2nd, so Frances will have to sit in the 3rd. So on and so forth, there will be 11 choices for Frances in total? Don't think about whether Mary or Frances will sit first, that is not a part of what I gave you. Determining the ordering of your objects within the segment is a different step. Use that formula in order to determine how many appropriately sized segments there are and once you've done that you can multiply by the number of ways to order your objects.

If you have a total of n spaces and you have x objects that must be next to each other then there are n- x-1 spots you can put them. Think about putting all x object in the first x spaces, that is the first spot you can put them. Now think about moving them over one space, that is the next spot you can put your x objects.

You can keep moving your group over by one until the last member of your set is located in the last available space.

Thus in the first configuration, the one where all your objects were jammed in the first x spaces, the last object was in the xth space. In the last configuration, where all your objects are jammed into the last x spaces, your last object is in the nth space. Correct me if I'm wrong, is this formula just counting the number of possible ways the xth object can be placed in n spaces?

It counts the ways you can select x adjacent spaces out of a total of n spaces. It does not count the ways you can place the objects within those x spaces, putting x specific objects in each of the spaces still requires you to multiply by x!. Use of this site constitutes acceptance of our User Agreement and Privacy Policy. Log in or sign up in seconds. Submit a new text post. Please try Google before posting. The title should be of the form "[Level and Discipline] General Topic.

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Become a Redditor and subscribe to one of thousands of communities. HomeworkHelp submitted 5 years ago by CommunistMao. Find the number of different ways they can sit in these 12 seats if i there are no restrictions ii Mary and Frances do not sit in seats which are next to each other, iii all 8 people sit together with no empty seats between them.

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