arthham

JEE Advanced 2014 · Permutations and Combinations

Six cards and six envelopes are numbered 1,2,3,4,5,61, 2, 3, 4, 5, 6 and cards are to be placed in envelopes so that each envelope contains exactly one card and no card is placed in the envelope bearing the same number and moreover the card numbered 11 is always placed in envelope numbered 22. Then the number of ways it can be done is

Answer: C. 5353

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Written solution

Given: cards 1,2,3,4,5,6 → envelopes 1,2,3,4,5,6 (one each)\text{Given: cards } 1,2,3,4,5,6 \ \to \ \text{envelopes } 1,2,3,4,5,6 \ \text{(one each)} no card i in envelope i\text{no card } i \text{ in envelope } i 1→envelope 2(fixed)1 \to \text{envelope } 2 \quad (\text{fixed}) To find: number of such arrangements\text{To find: number of such arrangements} 1. fix card 1→envelope 21.\ \text{fix card } 1 \to \text{envelope } 2 2. arrange the remaining 5 cards avoiding their own envelopes2.\ \text{arrange the remaining } 5 \text{ cards avoiding their own envelopes} 3. count with inclusion-exclusion3.\ \text{count with inclusion-exclusion} cards left: {2,3,4,5,6}\text{cards left: } \{2,3,4,5,6\} envelopes left: {1,3,4,5,6}\text{envelopes left: } \{1,3,4,5,6\} without restriction: 5!=120\text{without restriction: } 5! = 120 forbidden: card i→envelope i,i=3,4,5,6\text{forbidden: card } i \to \text{envelope } i, \quad i = 3,4,5,6 4 forbidden events4 \text{ forbidden events} N=5!−(41)4!+(42)3!−(43)2!+(44)1!N = 5! - \binom{4}{1}4! + \binom{4}{2}3! - \binom{4}{3}2! + \binom{4}{4}1! 5!=1205! = 120 (41)4!=4×24=96\binom{4}{1}4! = 4 \times 24 = 96 (42)3!=6×6=36\binom{4}{2}3! = 6 \times 6 = 36 (43)2!=4×2=8\binom{4}{3}2! = 4 \times 2 = 8 (44)1!=1\binom{4}{4}1! = 1 N=120−96+36−8+1N = 120 - 96 + 36 - 8 + 1 =24+36−8+1=53= 24 + 36 - 8 + 1 = 53 !6=265,2655=53 ✓!6 = 265, \quad \dfrac{265}{5} = 53 \ \checkmark 53⇒option (3)\boxed{53} \quad \Rightarrow \quad \text{option (3)}

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