Olympiad Maths Prep

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Problem 188

AMC 10/12, early questions
Number theory Difficulty 3.8 Find the answer

Arrange the positive even numbers into groups according to the following rule: (2)\left(2\right), (4,6)\left(4,6\right), (8,10,12)\left(8,10,12\right), (14,16,18,20)\left(14,16,18,20\right), \ldots, we call "44" the first number in the second group, "1616" the second number in the fourth group. If "20242024" is the mm-th number in the nn-th group, then m+n=______m+n=\_\_\_\_\_\_.

Official solution

To solve the problem, let's break down the solution step by step, closely following the original solution provided:

1. Identify the Position of 2024 in the Sequence of Even Numbers:
- Each positive even number can be expressed as 2k2k for some positive integer kk.
- Since 2024=2×10122024 = 2 \times 1012, it means 20242024 is the 10121012-th even number in the sequence of all even numbers.

2. **Determine the Group Number (nn) for 2024:**
- The sum of the first nn positive integers is given by the formula n(n+1)2\frac{n(n + 1)}{2}.
- The groups are formed by consecutive even numbers, with the 11st group having 11 number, the 22nd group having 22 numbers, and so on.
- We need to find the group (nn) such that the total count of numbers up to that group is just before or equal to 10121012.
- For n=44n = 44, the total count is (1+44)×442=990\frac{(1 + 44) \times 44}{2} = 990.
- For n=45n = 45, the total count is (1+45)×452=1035\frac{(1 + 45) \times 45}{2} = 1035.
- Therefore, 20242024 falls into the 4545-th group because 990<1012<1035990 < 1012 < 1035.

3. **Determine the Position (mm) of 2024 Within Its Group:**
- To find mm, we calculate how far 20242024 is from the start of its group.
- The difference between 10121012 and the total count up to the previous group (4444-th) is 1012990=221012 - 990 = 22.
- This means 20242024 is the 2222-nd number in the 4545-th group.

4. **Calculate m+nm + n:**
- With m=22m = 22 (position in the group) and n=45n = 45 (group number), we find m+n=45+22=67m + n = 45 + 22 = 67.

Therefore, the final answer, following the rules and format specified, is 67\boxed{67}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.