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.

Step-by-step 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 = 4 ! 2 ! 2 ! 3 ! 2 ! =18