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Permutations and Combinations question

2018 · 16 Apr · Shift 1 · Q33
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Permutations and Combinations question

2018 · 16 Apr · Shift 1 · Q33

JEE MainMathematicsPermutations and CombinationsMCQ+4 / −1
The number of numbers between 2,000 and 5,000 that can be formed with the digits 0, 1, 2, 3, 4 (repetition of digits is not allowed) and are multiple of 3 is :
  1. A
    24
  2. B
    30
  3. C
    36
  4. D
    48
View written solutionFree

Correct answer: B

  1. Digits available

    We can form 4-digit numbers using the digits {0,1,2,3,4}\{0,1,2,3,4\}{0,1,2,3,4} without repetition.

    We want numbers:

    • between 200020002000 and 500050005000
    • divisible by 333
  2. Condition for being between 2000 and 5000

    Since the number is 4-digit and must lie between 200020002000 and 500050005000, its thousands digit can only be: 2,3,42,3,42,3,4

  3. Condition for divisibility by 3

    A number is divisible by 333 if the sum of its digits is divisible by 333.

    The available digits are 0,1,2,3,40,1,2,3,40,1,2,3,4.

    Their residues modulo 333 are:

    • 0≡00 \equiv 00≡0
    • 1≡11 \equiv 11≡1
    • 2≡22 \equiv 22≡2
    • 3≡03 \equiv 03≡0
    • 4≡14 \equiv 14≡1

    We must choose 4 distinct digits from these 5 digits.

  4. Find which 4-digit selections give sum divisible by 3

    Total sum of all 5 digits is: 0+1+2+3+4=100+1+2+3+4=100+1+2+3+4=10 and 10≡1(mod3)10 \equiv 1 \pmod{3}10≡1(mod3)

    If we omit one digit ddd, then the sum of the chosen 4 digits is: 10−d10-d10−d For divisibility by 333: 10−d≡0(mod3)10-d \equiv 0 \pmod{3}10−d≡0(mod3) 1−d≡0(mod3)1-d \equiv 0 \pmod{3}1−d≡0(mod3) d≡1(mod3)d \equiv 1 \pmod{3}d≡1(mod3)

    Among {0,1,2,3,4}\{0,1,2,3,4\}{0,1,2,3,4}, the digits congruent to 1(mod3)1 \pmod{3}1(mod3) are: 1,41,41,4

    So the valid 4-digit sets are obtained by omitting either 111 or 444:

    • Set 1: {0,2,3,4}\{0,2,3,4\}{0,2,3,4}
    • Set 2: {0,1,2,3}\{0,1,2,3\}{0,1,2,3}
  5. Count arrangements from each valid set with first digit 2,3,2,3,2,3, or 444


    Case 1: Digits {0,2,3,4}\{0,2,3,4\}{0,2,3,4}

    Total permutations of these 4 digits: 4!=244! = 244!=24

    But numbers cannot start with 000. Number starting with 000: 3!=63! = 63!=6

    Hence valid 4-digit numbers from this set: 24−6=1824-6=1824−6=18

    All these have first digit among 2,3,42,3,42,3,4, so all lie between 200020002000 and 500050005000.


    Case 2: Digits {0,1,2,3}\{0,1,2,3\}{0,1,2,3}

    Total permutations: 4!=244! = 244!=24

    Numbers starting with 000: 3!=63! = 63!=6

    So 4-digit numbers formed: 24−6=1824-6=1824−6=18

    But to be between 200020002000 and 500050005000, first digit must be 222 or 333 only.

    Count directly:

    • First digit 222: remaining 3 digits can be arranged in 3!=63! = 63!=6 ways
    • First digit 333: remaining 3 digits can be arranged in 3!=63! = 63!=6 ways

    Total for this case: 6+6=126+6=126+6=12

  6. Total count

    18+12=3018+12=3018+12=30

  7. Compare with options

    The required number is: 30\boxed{30}30​

    So the correct option is B.

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