How many numbers can be formed with the digits 2, 3, 4, 5, 4, 3, 2 so that the odd digits occupy the odd places?

The table shows the places where the odd digits can be placed


Odd




odd




odd




Odd



There are 4 places


And 3 odd digits out of which 2 are of the same kind


Choose any 3 places out of the four places in 4C3 ways = 4 ways


In each way, the 3 digits can be placed in ways = 3 ways


total number of ways in which odd digits occupy odd places = = 12


Now there are 4 remaining digits out of which 2 are same of 1 kind, and 2 are same as another kind


they can be arranged in the remaining places in = 6 ways


total number of numbers where odd digit occupies odd places = 12 × 6 = 72


There are 72 such numbers


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