How many four digit numbers can be formed from the digits 1, 3, 5, 7 and 9 if no digit may be repeated in a number

Numbers: 1,2,4,5,7,8

There are 4 digits __ __ __ __

1st digit can be filled by any of the 6 numbers

Similarly 2nd, 3rd and 4th

Hence each digit will have 6 possibilities

Therefore no. of 4 digit numbers that can be formed are 6 x 6 x 6 x 6 = 1296