8 keys are no good there are only 6 possible keys left so the chance that the next one is good is 1/6
The next is 1 in 4 which means there are 4 keys left,
tries 4th last, no good :(
tries 3rd last, no good :(
tries second last, good great, no good doesn't matter, it has to be the last one.
So the worst scenario is that 3 more keys must be tried :)