2021 AMC 10A Problems/Problem 20

Problem

In how many ways can the sequence be rearranged so that no three consecutive terms are increasing and no three consecutive terms are decreasing?

Solution 1 (Bashing)

We write out the cases. These cases are the ones that work:

We count these out and get permutations that work.

~contactbibliophile

Solution 2 (Casework on the Consecutive Digits)

Reading the terms from left to right, we have two cases for the consecutive digits, where means increase and means decrease:

For note that for the second and fourth terms, one term must be and the other term must be or We have four sub-cases:

For the first two blanks must be and in some order, and the last blank must be for a total of possibilities. Similarly, also has possibilities.

For there are no restrictions for the numbers and So, we have possibilities. Similarly, also has possibilities.

Together, has possibilities. By symmetry, also has possibilities.

Finally, the answer is

Remark

This problem is a little similar to 2004 AIME I Problem 6.

~MRENTHUSIASM

Solution 3 (Similar to Solution 2)

Like Solution 2, we have two cases. Due to symmetry, we just need to count one of the cases. For the purpose of this solution, we will be doing . Instead of starting with 5, we start with 1.

There are two ways to place it:

_1_ _ _

_ _ _1_

Now we place 2, it can either be next to 1 and on the outside, or is place in where 1 would go in the other case. So now we have another two "sub case":

_1_2_(case 1)

21_ _ _(case 2)

There are 3! ways to arrange the rest for case 1, since there is no restriction.

For case 2, we need to consider how many ways to arrange 3,4,5 in a a>b<c fashion. It should seem pretty obvious that b has to be 3, so there will be 2! way to put 4 and 5.

Now we find our result, times 2 for symmetry, times 2 for placement of 1 and times (3!+2!) for the two different cases for placement of 2. This give us .

~~Xhte

Solution 4 (Symmetry)

We only need to find the # of rearrangements when 5 is the 4th digit and 5th digit. Find the total, and multiply by 2. Then we can get the answer by adding the case when 5 is the third digit.

Case : 5 is the 5th digit. __ __ __ __ 5

Then can only be either 1st digit or the 3rd digit.

4 __ __ __ 5, then the only way is that is the 3rd digit, so it can be either or , give us results.

__ __ 4 __ 5, then the 1st digit must be or , gives us way, and gives us ways. (Can't be because the first digit would increasing). Therefore, in the middle and in the last would result in ways.

Case : is the fourth digit. __ __ __ 5 __

Then the last digit can be all of the 4 numbers , , , and . Let's say if the last digit is , then the 2nd digit would be the largest for the remaining digits to prevent increasing order or decreasing order. Then the remaining two are interchangeable, give us ways. All of the can work, so case would result in ways.

Case : is in the middle. __ __ 5 __ __

Then there are only two cases: 1. , then 4 and 3 are interchangeable, which results in . Or it can be , then 4 and 2 are interchangeable, but it can not be , so there can only be 2 possible ways: , .

Therefore, case 3 would result in ways.

, so the total ways for case 1 and case 2 with both increasing and decreasing would be

Finally, we have

~Michael595

Video Solution by OmegaLearn (Using PIE - Principle of Inclusion Exclusion)

https://youtu.be/Fqak5BArpdc

~ pi_is_3.14

Video Solution by Power of Logic (Using Idea of Symmetrically Counting)

https://youtu.be/ZLQ8KYtai_M

Video Solution by TheBeautyofMath

https://youtu.be/UZZoSYHBJlI

~IceMatrix

See Also

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.