UPSC Prelims 2024·CSAT·Quantitative Aptitude·Combinatorics and Probability

How many distinct 8-digit numbers can be formed by rearranging the digits of the number 11223344 such that odd digits occupy odd positions and even digits occupy even positions?

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Last updated 23 May 2026, 3:31 pm IST
  1. A12
  2. B18
  3. C36Correct
  4. D72

Explanation

The number is 11223344. It has 8 digits: four odd digits (1, 1, 3, 3) and four even digits (2, 2, 4, 4). There are 8 positions in the 8-digit number. Odd positions are 1st, 3rd, 5th, 7th (4 positions). Even positions are 2nd, 4th, 6th, 8th (4 positions). Step 1: Arrange the odd digits (1, 1, 3, 3) in the 4 odd positions. This is a permutation of 4 items with repetitions. The formula is n! / (r1! * r2! * ...), where n is the total number of items, and r1, r2 are the counts of repeated items. Number of ways = 4! / (2! * 2!) = (4 * 3 * 2 * 1) / ((2 * 1) * (2 * 1)) = 24 / 4 = 6 ways. Step 2: Arrange the even digits (2, 2, 4, 4) in the 4 even positions. Similarly, this is a permutation of 4 items with repetitions. Number of ways = 4! / (2! * 2!) = (4 * 3 * 2 * 1) / ((2 * 1) * (2 * 1)) = 24 / 4 = 6 ways. Step 3: To find the total number of distinct 8-digit numbers, multiply the number of ways for arranging odd digits and even digits, as these are independent choices. Total distinct numbers = (Ways to arrange odd digits) * (Ways to arrange even digits) = 6 * 6 = 36. Therefore, 36 distinct 8-digit numbers can be formed. Option Analysis: A) 12: Incorrect. This would be 6+6 or 6*2, not the correct multiplication of independent arrangements. B) 18: Incorrect. C) 36: Correct, as calculated above. D) 72: Incorrect.
Quantitative Aptitude: How many distinct 8-digit numbers can be formed by rearranging the digits of the number 11223344 such that odd digits oc

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