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Permutations and Combinations question

2019 · 10 Apr · Shift 1 · Q34
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Permutations and Combinations question

2019 · 10 Apr · Shift 1 · Q34

JEE MainMathematicsPermutations and CombinationsMCQ+4 / −1
The number of 6 digit numbers that can be formed using the digits 0, 1, 2, 5, 7 and 9 which are divisible by 11 and no digit is repeated is :
  1. A
    36
  2. B
    60
  3. C
    72
  4. D
    48
View written solutionFree

Correct answer: B

  1. Divisibility rule for 11

For a 6-digit number with digits d1d2d3d4d5d6d_1d_2d_3d_4d_5d_6d1​d2​d3​d4​d5​d6​, the number is divisible by 111111 if

(d1+d3+d5)−(d2+d4+d6)(d_1+d_3+d_5) - (d_2+d_4+d_6)(d1​+d3​+d5​)−(d2​+d4​+d6​)

is a multiple of 111111.

Here the digits used are exactly 0,1,2,5,7,90,1,2,5,7,90,1,2,5,7,9 once each.

  1. Total sum of all digits
0+1+2+5+7+9=240+1+2+5+7+9 = 240+1+2+5+7+9=24

Let the sum of digits in odd positions be SoS_oSo​ and in even positions be SeS_eSe​. Then

So+Se=24S_o + S_e = 24So​+Se​=24

and divisibility by 111111 requires

So−Se=0,±11,±22S_o - S_e = 0, \pm 11, \pm 22So​−Se​=0,±11,±22

Since both So,SeS_o,S_eSo​,Se​ are sums of three of the given digits, the difference cannot be ±22\pm 22±22. Also, because 242424 is even, So−SeS_o-S_eSo​−Se​ must be even, so it cannot be ±11\pm 11±11. Hence the only possibility is

So−Se=0  ⟹  So=Se=12.S_o - S_e = 0 \implies S_o=S_e=12.So​−Se​=0⟹So​=Se​=12.

So we need to split the 6 digits into two groups of 3 each, each summing to 121212.

  1. Find 3-digit subsets with sum 12

From {0,1,2,5,7,9}\{0,1,2,5,7,9\}{0,1,2,5,7,9}, the 3-element subsets summing to 121212 are:

  • {0,5,7}\{0,5,7\}{0,5,7}
  • {1,2,9}\{1,2,9\}{1,2,9}

These are complementary, so one must occupy the odd positions and the other the even positions.

Thus there are 2 ways to choose which set goes to odd positions:

  • odd positions: {0,5,7}\{0,5,7\}{0,5,7}, even positions: {1,2,9}\{1,2,9\}{1,2,9}
  • odd positions: {1,2,9}\{1,2,9\}{1,2,9}, even positions: {0,5,7}\{0,5,7\}{0,5,7}
  1. Arrange digits in positions, respecting first digit nonzero

We now count arrangements for each case.


Case 1: Odd positions get {0,5,7}\{0,5,7\}{0,5,7}, even positions get {1,2,9}\{1,2,9\}{1,2,9}

Odd positions are 1,3,51,3,51,3,5. Since the first digit cannot be 000, among the 3!=63! = 63!=6 arrangements of {0,5,7}\{0,5,7\}{0,5,7} in odd positions, those with 000 at position 1 are invalid.

If 000 is fixed at position 1, the remaining two digits can be arranged in 2!=22! = 22!=2 ways. So valid arrangements for odd positions:

6−2=46-2 = 46−2=4

Even positions can be filled with {1,2,9}\{1,2,9\}{1,2,9} in

3!=63! = 63!=6

ways.

Hence total for Case 1:

4×6=244 \times 6 = 244×6=24

Case 2: Odd positions get {1,2,9}\{1,2,9\}{1,2,9}, even positions get {0,5,7}\{0,5,7\}{0,5,7}

Now the first digit is one of 1,2,91,2,91,2,9, so no restriction from leading zero.

Odd positions can be arranged in

3!=63! = 63!=6

ways.

Even positions can be arranged in

3!=63! = 63!=6

ways.

Hence total for Case 2:

6×6=366 \times 6 = 366×6=36
  1. Total count
24+36=6024 + 36 = 6024+36=60

Therefore, the number of such 6-digit numbers is

60\boxed{60}60​

So the correct option is B.

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