find the probability that the five digit number using 1,2,5,6 and 8 would be divisible by 4
To be divisible by 4, the last two digits must be a multiple of 4. Using those digits, the only possibilities are
12,16,28,56,88
There are 5^5 possible 5-digit numbers. Of those,
5^3 * 5 = 5^4 end in those digits.
So, 5^4/5^5 = 1/5 of the numbers will be divisible by 4.
Adjust that if no repetitions are allowed.