Equal Sum Partition in Odd Series

CAT 2025 Slot 1 · Quantitative Ability · Easy · Algebra

In the set of consecutive odd numbers {1,3,5,…,57}\{1, 3, 5, \dots, 57\}, there is a number kk such that the sum of all the elements less than kk is equal to the sum of all the elements greater than kk. Then, kk equals

  1. A.

    43

  2. B.

    37

  3. C.

    39

  4. D.

    41

Answer

D

Explanation

Let kk be the mm-th term of the sequence of consecutive odd numbers, so k=2m−1k = 2m - 1. The sum of elements less than kk is the sum of the first m−1m-1 odd numbers, which is (m−1)2(m-1)^2. The sum of all odd numbers from 11 to 5757 (which has 2929 terms) is 292=84129^2 = 841. We are given: Sum(<k)=Sum(>k)=S\text{Sum}(< k) = \text{Sum}(> k) = S 2S+k=841  ⟹  2(m−1)2+(2m−1)=8412S + k = 841 \implies 2(m-1)^2 + (2m - 1) = 841 2(m2−2m+1)+2m−1=841  ⟹  2m2−2m+1=841  ⟹  2m2−2m−840=02(m^2 - 2m + 1) + 2m - 1 = 841 \implies 2m^2 - 2m + 1 = 841 \implies 2m^2 - 2m - 840 = 0 m2−m−420=0  ⟹  (m−21)(m+20)=0  ⟹  m=21m^2 - m - 420 = 0 \implies (m - 21)(m + 20) = 0 \implies m = 21 Thus, k=2(21)−1=41k = 2(21) - 1 = 41.

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