First slide

Combinations

Question

There are 4n things of which n are alike and all the rest different. Then the number of permutations of 4n things taken 2n at a time, each permutation containing the n like things

Moderate

A

$\frac{4 n!}{{(n!)}^{2}}$
B

$\frac{4 n!}{2 n!}$
C

$\frac{4 n!}{n!}$
D

$\frac{3 n!}{{(n!)}^{2}}$

Solution

${}^{3 n}C_{n} \times \frac{(2 n)!}{n!} = \frac{3 n!}{(n!)^{2}} (Selection and Arrangement)$

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