Question

The number of numbers of seven digits that can be formed with the digits 1,2,3,4,3,2,1 so that odd digits always occupy odd places is

• A. 18
• B. 36
• C. 72
• D. 144

Answer 1

The correct option is A 18

Number. Of odd places 4 odd digits (1,3,3,1) Number of even places 3 even digits (2,4,2) $\therefore$ Required number $=\frac{4!}{2!2!}\frac{3!}{2!}=2\times 3\times 3=18$