I cant explain it via computer....sorry! but the answer is there are four combinations with at 25% probability (equal) of getting each combination. Play around and see if you can come up with that answer.
Sorry i can't help more!
look at all the possible combinations of E, e, G, g, B, b or what ever it is... write them down... you will find that there are only 4 possibilities which occur the same amount of times - therefore there is a 25% probability of each (as swordfish said!)