3cars are parked on a drive. they are red, blue and green. list all
the different orders they could be parked in. what is the probability
that the green car is first?

One way you could do it is by using the roster or list meathod which is normaly long and tedius. 
RBG
RGB
GRB
GBR
BRG
BGR
so six possibilities

the easier way is to use permutations also nicknamed the slot meathod by my teacher
so in the first slot there are 3 choices
in the 2nd slot there are 2 choices since one was already taken up by the 1st slot
3rd slot 1 choice because 1 was taken by slot 2
so 3 times 2 times 1=6 or six posssiblities

So if we look at our roster/list, 2 out of 6 times or 2/6 or 1/3 of the permutations the Green car was first