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Permutations and Combinations question

2024 · 8 Apr · Shift 2 · Q46
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Permutations and Combinations question

2024 · 8 Apr · Shift 2 · Q46

JEE MainMathematicsPermutations and CombinationsMCQ+4 / −1
The number of ways five alphabets can be chosen from the alphabets of the word MATHEMATICS, where the chosen alphabets are not necessarily distinct, is equal to:
  1. A
    179
  2. B
    177
  3. C
    175
  4. D
    181
View written solutionFree

Correct answer: A

  1. Count the letters available in MATHEMATICS

    The word MATHEMATICS has the following letter multiplicities:

    • MMM appears 222 times
    • AAA appears 222 times
    • TTT appears 222 times
    • and H,E,I,C,SH,E,I,C,SH,E,I,C,S each appear 111 time

    So there are 888 distinct types of letters: M,A,T,H,E,I,C,SM,A,T,H,E,I,C,SM,A,T,H,E,I,C,S

  2. Interpret the question

    We must choose 555 alphabets from these letters, with the condition that chosen alphabets are not necessarily distinct, but we cannot exceed the available multiplicity in the word.

    So we need the number of nonnegative integer solutions of m+a+t+h+e+i+c+s=5m+a+t+h+e+i+c+s=5m+a+t+h+e+i+c+s=5 subject to m,a,t≤2,h,e,i,c,s≤1.m,a,t\le 2, \qquad h,e,i,c,s\le 1.m,a,t≤2,h,e,i,c,s≤1.

  3. Casewise counting based on repetitions

    Since only M,A,TM,A,TM,A,T can repeat, possible patterns for choosing 555 letters are:

    Case 1: All 555 chosen letters are distinct

    We choose any 555 distinct letter-types from the 888 available: (85)=56\binom{8}{5}=56(58​)=56

    Case 2: Exactly one letter is repeated twice

    The repeated letter must be one of M,A,TM,A,TM,A,T.

    • Choose the repeated letter: (31)=3\binom{3}{1}=3(13​)=3
    • Choose the remaining 333 distinct letters from the other 777 letter-types: (73)=35\binom{7}{3}=35(37​)=35

    Total: 3⋅35=1053\cdot 35=1053⋅35=105

    Case 3: Exactly two letters are repeated twice each

    Then the pattern is 2+2+12+2+12+2+1.

    The two repeated letters must be chosen from M,A,TM,A,TM,A,T: (32)=3\binom{3}{2}=3(23​)=3

    Then choose the remaining single letter from the other 666 letter-types: (61)=6\binom{6}{1}=6(16​)=6

    Total: 3⋅6=183\cdot 6=183⋅6=18

  4. Add all cases

    56+105+18=17956+105+18=17956+105+18=179

  5. Conclusion

    The required number of ways is 179\boxed{179}179​

  6. Option check

    • A: 179179179 ✅
    • B: 177177177 ❌
    • C: 175175175 ❌
    • D: 181181181 ❌

Therefore, the correct option is A.

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