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
Write and
for the Steelie and the Tiger respectively. Putting in
and
first, in order to avoid them being next to each other, we must have the arrangement
,
,
, or any of these with the
and
swapped. This gives
ways, and there are then
ways to put in the Aggie and Bumblebee, for a total of
.
Solution 2 (complementary counting)
There would be ways to arrange the
marbles, except for the condition that the Steelie and Tiger cannot be next to each other. If we did place them next to each other with the Steelie first, there would be
ways to place them (namely
,
; or
, where
and
denote the Steelie and the Tiger as in Solution 1). Accounting for the other possible order, there are a total of
ways. Now, there are
ways to place
and
, giving overall
ways to arrange the marbles with
and
next to each other. Subtracting this from
(to remove the cases which are not allowed) gives
valid ways to arrange the marbles.
Solution 3 (variant of Solution 2)
As in Solution 2, there are total ways to arrange the marbles without any constraints. To count the number of ways where the Steelie and the Tiger are next to each other, we treat them together as a "super marble". There are
ways to arrange the Steelie and Tiger within the super marble, then
ways to put the super marble in a row with the Aggie and the Bumblebee. Thus the answer is
.
Solution 4(Using Probability)
To solve this problem, we can first consider the chance of this happening and try to involve it in the total possibilities. As we know, we do not want the Steelie and the Tiger together, the total outcomes are that they are together and that they are not together. As seen in the previous solutions, them being together and not together total possibilities with them both being
separate ways. To solve this problem, we must consider the possibility of this happening and multiply it by the total ways to get an accurate result. The probability of this happening would be
since they can either be together or not. This is also seen in Pascal's Triangle as the sum of the numbers in each row result in powers of
, where each entity can be a part of the total or not(for committees) but this also applies to general counting as well. The total number of ways without restrictions is
, and by multiplying it by
, we get
.
Video Solution
See also
2020 AMC 8 (Problems • Answer Key • Resources) | ||
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Followed by Problem 11 | |
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