2024 AMC 10A Problems/Problem 20
Contents
[hide]Problem
Let be a subset of such that the following two conditions hold:
- If and are distinct elements of , then
- If and are distinct odd elements of , then .
What is the maximum possible number of elements in ?
Video Solution by Scholars Foundation
https://www.youtube.com/watch?v=FKOqZau--5w&t=1s
Solution 1
All lists are organized in ascending order:
By listing out the smallest possible elements of subset we can find that subset starts with It is easily noticed that the elements of the subset "loop around" every 3 elements, specifically adding 10 each time. This means that there will be or whole loops in the subset implying that there will be elements in S. However, we have undercounted, as we did not count the remainder that resulted from With a remainder of we can fit more elements into the subset namely and resulting in a total of or elements in subset .
NOTE:
To prove that this is the best we can do, consider adding each element one by one, for the first element, say n. If n is greater than 2, we can choose n - 2 which is always better. Therefore, n = 1 or n = 2.
If n = 2 was optimal, then choose it, then the set of usable numbers in becomes 5 through 2024. We can transform the usable set of to where contains the numbers 1 through 2020. Because we assumed n = 2 was optimal, we can choose n = 2 for the set too. Because every element in is 4 below the elements of , choosing 2 in would mean choosing 6 in set . By induction we see that our list would be {2,6,10,14,18,.....2022} which only gives 506 elements which is sub-optimal. Therefore, we can conclude that n = 1 is optimal, and we proceed as the solution above.
-weihou0
Solution 2
Notice that we can first place odd numbers, then place even numbers between each pair. We can start at and continue from there. Realize that the smallest number such that reproduces odd number is . The next ones are . We can proceed to find the number of numbers in this particular sequence. From the equation , we get that works, so this means there is 253 solutions. Looking at we can see that there could only be 1 possible number between each pair, yielding . Then see that we can fit two more into the number count since the set to can fit two evens. Now this means and don’t work. Now test out . Using the same method, we get that is the maximum number in the set. Everything above doesn’t work, as we can split it down into smaller subgroups, so the answer is .
~EaZ_Shadow
Video Solution by Power Solve
https://www.youtube.com/watch?v=NZ0SBMqeAfg
Video Solution by Pi Academy
https://youtu.be/fW7OGWee31c?si=oq7toGPh2QaksLHE
Video Solution by SpreadTheMathLove
https://www.youtube.com/watch?v=6SQ74nt3ynw
See also
2024 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 19 |
Followed by Problem 21 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
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