How many numbers can be formed using digits 1,2 3 4 3 2 1 such that even digits always occupy even places?

How many numbers can be formed with the digits 1, 2, 3, 4, 3, 2, 1 so that the odd digits always occupy the odd places?

There are 4 odd digits (1,3,3 and 1) that are to be arranged in 4 odd places in\[\frac{4!}{2!2!}\]ways.The remaining 3 even digits 2, 2 and 4 can be arranged in 3 even places in\[\frac{3!}{2!}\]ways.By fundamental principle of counting:

Required number of arrangements =\[\frac{4!}{2!2!}\]\[\times\]\[\frac{3!}{2!}\]= 18

Concept: Factorial N (N!) Permutations and Combinations

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