.
How many different 4-digit even numbers can be formed from the digits 1,3,5,6,8, and 9
if no repetition of digits is allowed?
~~~~~~~~~~~~~~~~~
For the last (ones) digit, you have only two options: you must have it either 6 or 8.
For the 1-st (leftmost) digit, you have any of remaining 5 digits (from original 6 digits);
For the next, 2-nd digit, you have any of remaining 4 digits;
For the next, 3-rd digit, you have any of remaining 3 digits.
Thus, in all, you have 2*5*4*3 = 120 options and 120 possible numbers, as described.
Solved.