28. How many 4-digit positive multiples of 4 can be formed from the digits $0,1,2,3,4,5,6$ such that each digit appears without repetition?
28. Answer: 208
Note that a positive integer $k$ is a multiple of 4 if and only if the number formed by the last two digits of $k$ (in the same order) is a multiple of 4 . There are 12 possible multiples of 4 that can be formed from the digits $0,1,2,3,4,5,6$ without repetition, namely
$$
20,40,60,12,32,52,04,24,64,16,36,56 \text {. }
$$

If 0 appears in the last two digits, there are 5 choices for the first digit and 4 choices for the second digit. But if 0 does not appear, there are 4 choices for the first digit and also 4 choices for the second digit. Total number is
$$
4 \times 5 \times 4+8 \times 4 \times 4=208 \text {. }
$$