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Miscellaneous Exercise · Q5

Q.How many 5 letter word can be formed using 3 letters of the word ALGORITHM and 2 letters from the word DUES.

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Pick 3 of the 9 distinct letters of ALGORITHM and 2 of the 4 distinct letters of DUES (the two words share no common letters), then arrange all 5 chosen letters, which are all different from one another.

Selection: nCr=n!r!(n−r)!^nC_r=\dfrac{n!}{r!(n-r)!}. Arrangement of rr distinct items in a row: r!r!.

  1. ALGORITHM ={A,L,G,O,R,I,T,H,M}=\{A,L,G,O,R,I,T,H,M\} — 9 letters, all distinct. Choose 3 of them: 9C3=9×8×73×2×1=5046=84^9C_3=\dfrac{9\times8\times7}{3\times2\times1}=\dfrac{504}{6}=84 ways.
  2. DUES ={D,U,E,S}=\{D,U,E,S\} — 4 letters, all distinct, and none of D, U, E, S coincides with any letter of ALGORITHM. Choose 2 of them: 4C2=4×32×1=6^4C_2=\dfrac{4\times3}{2\times1}=6 ways.
  3. Total ways to select the 5 letters (3 from ALGORITHM, 2 from DUES) =84×6=504=84\times6=504. …

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