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Permutations and Combinations question

2020 · 2 Sep · Shift 1 · Q27
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Permutations and Combinations question

2020 · 2 Sep · Shift 1 · Q27

JEE MainMathematicsPermutations and CombinationsNumerical+4 / −1
If the letters of the word 'MOTHER' be permuted and all the words so formed (with or without meaning) be listed as in a dictionary, then the position of the word 'MOTHER' is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 309

We need the dictionary rank of the word MOTHER among all permutations of its letters.

The letters are: M,O,T,H,E,RM, O, T, H, E, RM,O,T,H,E,R

Arrange them in alphabetical order: E<H<M<O<R<TE < H < M < O < R < TE<H<M<O<R<T

Since all letters are distinct, we count how many valid permutations come before MOTHER in dictionary order.


Step 1: First letter = M

We count words beginning with a letter smaller than MMM.

Smaller letters available: E,HE, HE,H

For each such choice, the remaining 555 letters can be arranged in: 5!=1205! = 1205!=120

So count: 2×120=2402 \times 120 = 2402×120=240


Step 2: Fix first letter M, second letter = O

Remaining letters are: E,H,O,R,TE, H, O, R, TE,H,O,R,T

Now count words beginning with MMM and having second letter smaller than OOO.

Smaller available letters than OOO: E,HE, HE,H

For each such choice, remaining 444 letters can be arranged in: 4!=244! = 244!=24

So count: 2×24=482 \times 24 = 482×24=48

Running total: 240+48=288240 + 48 = 288240+48=288


Step 3: Fix MO, third letter = T

Remaining letters are: E,H,R,TE, H, R, TE,H,R,T

Now count words beginning with MOMOMO and having third letter smaller than TTT.

Smaller available letters than TTT: E,H,RE, H, RE,H,R

For each such choice, remaining 333 letters can be arranged in: 3!=63! = 63!=6

So count: 3×6=183 \times 6 = 183×6=18

Running total: 288+18=306288 + 18 = 306288+18=306


Step 4: Fix MOT, fourth letter = H

Remaining letters are: E,H,RE, H, RE,H,R

Letters smaller than HHH: only EEE

For this choice, remaining 222 letters can be arranged in: 2!=22! = 22!=2

So count: 1×2=21 \times 2 = 21×2=2

Running total: 306+2=308306 + 2 = 308306+2=308


Step 5: Fix MOTH, fifth letter = E

Remaining letters are: E,RE, RE,R

Letters smaller than EEE: none

So contribution: 000


Step 6: Fix MOTHE, sixth letter = R

Only one letter remains, so no additional permutations come before it.


Step 7: Add 1 for the word itself

So rank of MOTHER is: 308+1=309308 + 1 = 309308+1=309


Final Answer

The position of the word MOTHER is: 309\boxed{309}309​

The derived answer matches the stored correct answer.

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