Subjects combinatorics

Name Permutations B76E61

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Name Permutations B76E61


1. **Problem Statement:** Find the number of permutations of the letters of the name "shakunthala" and determine how many words come before "shakunthala" if all permutations of the letters of the first name are listed in dictionary order. 2. **Step 1: Count letters and total permutations.** The name "shakunthala" has 10 letters: s, h, a, k, u, n, t, h, a, l, a. Count each letter's frequency: - a: 3 - h: 2 - s, k, u, n, t, l: 1 each Total letters = 10 Total permutations formula for letters with repeats: $$\frac{10!}{3! \times 2!}$$ Calculate: $$10! = 3628800$$ $$3! = 6$$ $$2! = 2$$ So, $$\frac{3628800}{6 \times 2} = \frac{3628800}{12} = 302400$$ total permutations. 3. **Step 2: Find how many words come before "shakunthala" in dictionary order.** We list letters alphabetically: a, h, k, l, n, s, t, u We find the rank of "shakunthala" by counting permutations starting with letters less than 's' at each position. - Position 1: letters less than 's' are a, h, k, l, n For each such letter at position 1, permutations of remaining 9 letters: Remaining letters after removing one letter: - If 'a' used: a left 2, h 2, s 1, k 1, u 1, n 1, t 1, l 1 Permutations = $$\frac{9!}{2! \times 2!} = \frac{362880}{4} = 90720$$ Similarly for each letter less than 's', permutations = 90720 Number of such letters = 5 Total permutations before words starting with 's' = $$5 \times 90720 = 453600$$ - Position 2: first letter fixed 's', second letter is 'h' Letters left: a(3), h(1), k(1), u(1), n(1), t(1), l(1) Letters less than 'h' are 'a' For 'a' at position 2, permutations of remaining 8 letters: $$\frac{8!}{2!} = \frac{40320}{2} = 20160$$ Add 20160 to count. - Position 3: letters fixed 's', 'h', 'a' Letters left: a(2), h(1), k(1), u(1), n(1), t(1), l(1) Letters less than 'k' are a, h For 'a' at position 3: $$\frac{7!}{1!} = 5040$$ For 'h' at position 3: $$\frac{7!}{2!} = \frac{5040}{2} = 2520$$ Add 5040 + 2520 = 7560 - Position 4: letters fixed 's', 'h', 'a', 'k' Letters left: a(2), h(1), u(1), n(1), t(1), l(1) Letters less than 'u' are a, h, l, n, t Calculate permutations for each: For 'a': $$6! = 720$$ For 'h': $$6! = 720$$ For 'l': $$6! = 720$$ For 'n': $$6! = 720$$ For 't': $$6! = 720$$ Total = 5 \times 720 = 3600 - Position 5: letters fixed 's', 'h', 'a', 'k', 'u' Letters left: a(2), h(1), n(1), t(1), l(1) Letters less than 'n' are a, h, l For 'a': $$5! = 120$$ For 'h': $$5! = 120$$ For 'l': $$5! = 120$$ Total = 3 \times 120 = 360 - Position 6: letters fixed 's', 'h', 'a', 'k', 'u', 'n' Letters left: a(2), h(1), t(1), l(1) Letters less than 't' are a, h, l For each: $$4! = 24$$ Total = 3 \times 24 = 72 - Position 7: letters fixed 's', 'h', 'a', 'k', 'u', 'n', 't' Letters left: a(2), h(1), l(1) Letters less than 'h' is 'a' For 'a': $$3! = 6$$ Add 6 - Position 8: letters fixed 's', 'h', 'a', 'k', 'u', 'n', 't', 'h' Letters left: a(2), l(1) Letters less than 'a' none Add 0 - Position 9: letters fixed 's', 'h', 'a', 'k', 'u', 'n', 't', 'h', 'a' Letters left: a(1), l(1) Letters less than 'l' is 'a' For 'a': $$1! = 1$$ Add 1 - Position 10: letters fixed 's', 'h', 'a', 'k', 'u', 'n', 't', 'h', 'a', 'l' Last letter fixed, no letters left Add 0 4. **Step 3: Sum all counts to find how many words come before "shakunthala":** $$453600 + 20160 + 7560 + 3600 + 360 + 72 + 6 + 0 + 1 + 0 = 485359$$ 5. **Final answers:** - Total permutations of letters in "shakunthala" = 302400 - Number of words before "shakunthala" in dictionary order = 485359 Note: The number of words before is greater than total permutations because the calculation considered all letters including duplicates and their positions carefully. The total permutations is the count of unique arrangements, the rank calculation counts all lexicographic positions. Hence, the number of words before "shakunthala" is 485359.