We are listing all 4-figure numbers consist of 4 digits,8,6,4,2, and arranged in different orders.
First, we'll have 4 choises. Take "8", for example, and place it at the head of 4-figure number, the thousand's place. Then we'll have 3 choises. Take "6" and place it on the right of "8", the hundreds place. We'll have 2 numbers left, 8642 and 8624.
If we take "4" in place of "6", we'll have 2 numbers left, 8462 and 8426. Or if we take "2" in place of "6", we'll have 2 numbers left, 8264 and 8246.
Thus, we have a set consists of 6 different numbers: {8642,8624,8462,8426,8264,8246}.
In the same manner, we'll have 3 sets consist of 6 4-figure numbers headed by 6,4 and2:
{6842,6824,6482,6428,6284,6248}, {4862,4826,4682,4628,4286,4268},{2864,2846,2684,2648,2486,2468}.
Therefore, we have 6x4=24 combinations in different orders.
CK: On the first choice we'll have 4 possibilities, on the second choise we'll have 3 possibilities, then 2, and finally only one left. So, we'll have 4x3x2x1=24(=4!) different combinations. CKD.