Once you attempt the question then PrepInsta explanation will be displayed.

consider 4 digits _ _ _ _

1st blank can be filled in 7C1 ways (0,3,6 are neglected as the first digit should

not be 0)

2st blank can be filled in 8C1 ways (0 considered along with 1,2,4,5,7,8,9)

3st blank can be filled in 8C1 ways

4st blank can be filled in 8C1 ways

Therefore total 4 digit number without 3 and 6 is 7 x 8 x 8 x 8=3584