csathard

The digits 1 to 9 are arranged in three rows in such a way that each row contains three digits, and the number formed in the second row is twice the number formed in the first row; and the number formed in the third row is thrice the number formed in the first row. Repetition of digits is not allowed. If only three of the four digits 2, 3, 7 and 9 are allowed to use in the first row, how many such combinations are possible to be arranged in the three rows?

  1. A.40
  2. B.3
  3. C.2
  4. D.1
▶ Answer & Explanation

Correct answer: C. 2

Let the number formed in the first row be X. The numbers formed in the second and third rows are 2X and 3X respectively. All digits from 1 to 9 must be used exactly once across the three rows. The sum of digits 1 through 9 is 45. The sum of the numbers X, 2X, and 3X is 6X. For X to be a three-digit number, 2X and 3X must also be three-digit numbers. The sum of the digits of X, 2X, and 3X, when treated as numbers, must be divisible by 9 if the digits are a permutation of 1-9. However, the condition is that the digits themselves are unique and form these numbers. The sum of the digits used (1-9) is 45. The sum of the three numbers (X + 2X + 3X) must be 6X. If X is a three-digit number, the smallest possible sum of digits from 1-9 is 1+2+3 = 6 and the largest is 7+8+9 = 24 for each row. The total number formed by concatenating the digits of X, 2X, and 3X must be a permutation of 1-9. Considering the constraints and the possible values for X such that 2X and 3X are also 3-digit numbers using unique digits from 1-9, only specific sets of X are possible. The problem further restricts the digits available for the first row (X) to a subset of {2, 3, 7, 9}. By testing these combinations, it's found that only two valid arrangements satisfy all conditions.

Source: UPSC csat 2022

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