n4 = number of elements in S4 = $${}^{10}{C_4}$$ = 10 $$\times$$ 9 $$\times$$ 8 $$\times$$ 7
$$ \Rightarrow {{{n_4}} \over {12}} = 420$$
2
IIT-JEE 1998
MCQ (More than One Correct Answer)
An n-digit number is a positive number with exactly digits. Nine hundred distinct n-digit numbers are to be formed using only the three digits 2, 5 and 7. The smallest value of n for which this is possible is
A
6
B
7
C
8
D
9
Questions Asked from Permutations and Combinations
On those following papers in MCQ (Multiple Correct Answer)
Number in Brackets after Paper Indicates No. of Questions