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Permutations and Combinations question

2019 · 8 Apr · Shift 1 · Q42
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Permutations and Combinations question

2019 · 8 Apr · Shift 1 · Q42

JEE MainMathematicsPermutations and CombinationsMCQ+4 / −1
All possible numbers are formed using the digits 1, 1, 2, 2, 2, 2, 3, 4, 4 taken all at a time. The number of such numbers in which the odd digits occupy even places is :
  1. A
    175
  2. B
    162
  3. C
    160
  4. D
    180
View written solutionFree

Correct answer: D

  1. Identify the digits and their positions

    The digits are: 1,1,2,2,2,2,3,4,41,1,2,2,2,2,3,4,41,1,2,2,2,2,3,4,4

    Total digits =9=9=9, so each number formed has 999 places.

    Odd digits are: 1,1,31,1,31,1,3 There are 333 odd digits.

    Even places among 999 positions are: 2,4,6,82,4,6,82,4,6,8 So there are 444 even positions.

  2. Condition: odd digits occupy even places

    This means all the odd digits 1,1,31,1,31,1,3 must be placed in even positions only.

    So first, choose 333 of the 444 even positions for these odd digits: (43)=4\binom{4}{3}=4(34​)=4

  3. Arrange the odd digits in the chosen even positions

    We must arrange 1,1,31,1,31,1,3 in those 333 selected places.

    Number of distinct arrangements: 3!2!=3\frac{3!}{2!}=32!3!​=3

    So, total ways to place odd digits: 4×3=124 \times 3 = 124×3=12

  4. Arrange the remaining even digits in the remaining places

    After placing the odd digits, the remaining digits are: 2,2,2,2,4,42,2,2,2,4,42,2,2,2,4,4 These occupy the remaining 666 positions.

    Number of distinct arrangements of these 666 digits: 6!4!2!=72024⋅2=15\frac{6!}{4!2!} = \frac{720}{24\cdot 2}=154!2!6!​=24⋅2720​=15

  5. Total number of required numbers

    Multiply the two independent counts: 12×15=18012 \times 15 = 18012×15=180

  6. Check options

    The correct value is: 180180180

    Hence, the correct option is D.

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