2020 AMC 8 Problems/Problem 10
Contents
Problem
Zara has a collection of marbles: an Aggie, a Bumblebee, a Steelie, and a Tiger. She wants to display them in a row on a shelf, but does not want to put the Steelie and the Tiger next to one another. In how many ways can she do this?
Solution 1
By the Georgeooga-Harryooga Theorem there are way to arrange the marbles.
Solution 2
Let the Aggie, Bumblebee, Steelie, and Tiger, be referred to by and , respectively. If we ignore the constraint that and cannot be next to each other, we get a total of ways to arrange the 4 marbles. We now simply have to subtract out the number of ways that and can be next to each other. If we place and next to each other in that order, then there are three places that we can place them, namely in the first two slots, in the second two slots, or in the last two slots (i.e. ). However, we could also have placed and in the opposite order (i.e. ). Thus there are 6 ways of placing and directly next to each other. Next, notice that for each of these placements, we have two open slots for placing and . Specifically, we can place in the first open slot and in the second open slot or switch their order and place in the first open slot and in the second open slot. This gives us a total of ways to place and next to each other. Subtracting this from the total number of arrangements gives us total arrangements .
We can also solve this problem directly by looking at the number of ways that we can place and such that they are not directly next to each other. Observe that there are three ways to place and (in that order) into the four slots so they are not next to each other (i.e. ). However, we could also have placed and in the opposite order (i.e. ). Thus there are 6 ways of placing and so that they are not next to each other. Next, notice that for each of these placements, we have two open slots for placing and . Specifically, we can place in the first open slot and in the second open slot or switch their order and place in the first open slot and in the second open slot. This gives us a total of ways to place and such that they are not next to each other .
~junaidmansuri
Solution 3
Let's try complementary counting. There ways to arrange the 4 marbles. However, there are arrangements where Steelie and Tiger are next to each other. (Think about permutations of the element ST, A, and B or TS, A, and B). Thus,
Solution 4
We use complementary counting: we will count the numbers of ways where Steelie and Tiger are together and subtract that from the total count. Treat the Steelie and the Tiger as a "super marble." There are ways to arrange Steelie and Tiger within this "super marble." Then there are ways to arrange the "super marble" and Zara's two other marbles in a row. Since there are ways to arrange the marbles without any restrictions, the answer is given by
-franzliszt
Solution 5(proof of Georgeooga-Harryooga Theorem used in solution 1)
We will use the following
The Georgeooga-Harryooga Theorem states that if you have distinguishable objects and of them cannot be together, then there are ways to arrange the objects.
Let our group of objects be represented like so , , , ..., , . Let the last objects be the ones we can't have together.
Then we can organize our objects like so .
We have ways to arrange the objects in that list.
Now we have blanks and other objects so we have ways to arrange the objects we can't put together.
By fundamental counting principal our answer is .
Proof by RedFireTruck (talk) 12:09, 1 February 2021 (EST)
Back to the problem. By the Georgeooga-Harryooga Theorem, our answer is .
-franzliszt
Video Solution
See also
2020 AMC 8 (Problems • Answer Key • Resources) | ||
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Followed by Problem 11 | |
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