How many 7 digit numbers using all digits from 1, 1, 2, 2, 3, 3, 4, so that odd digit occupy the odd places :

Options

A. 16
B. 18
C. 21
D. 24

Answer

B

Explanation

`(I)/(O) , (II)/(.), (III)/(O), (IV)/(.),(V)/(O) , (IV)/(.), (VII)/(O) : {,1,1,2,2,3,3,4}
` ((4!)/(2! Xx 2!)) xx (3!)/(2!) = 18`