The numbers can be formed using the digit $1,2,3,4,3,2,1$ so that odd digits always occupy odd places in ways.
The numbers can be formed using the digit $1,2,3,4,3,2,1$ so that odd digits always occupy odd places in ways.
9
18
6
3
Solution
We have 4 odd digits i.e. 1,1,3,3 and 3 even digits i.e. 2,2, 4 In a 7 digit number, there are 4 odd and 3 even places.
So number of possible ways $=\frac{4 !}{2 ! 2 !} \times \frac{3 !}{2 !}=18$
