Question

There are 7 greetings cards, each of a different colour and 7 envelopes of same 7 colours as that of the cards. The number of ways in which the cards can be put in envelopes, so that exactly 4 of the cards go into envelopes of respective colour is,
(A) ${ }^{7} \mathrm{C}_{3}$
(B) $2 .{ }^{7} \mathrm{C}_{3}$
(C) $3 !{ }^{4} \mathrm{C}_{4}$
(D) $3 !^{7} \mathrm{C}_{3}{ }^{4} \mathrm{C}_{3}$

Answer 1

Ans: (B)
Hint $:{ }^{7} C_{4} \times 3 !\left(\frac{1}{2 !}-\frac{1}{3 !}\right)={ }^{7} C_{3} \times 2$

Answer 2

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Answer 3

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