Athree character computer code consist of two letters. what is the probability that the code will be aba? what is the probability that the code has repeated letters. either letter can be repeated

Well if you write out all the possibilities, it would look like this: 
AAB
ABB
ABA
BBA
BAB
BAA 
^ That is just assuming that you can't do BBB & AAA. 

With that, the probability of getting ABA is 1/6 because there are 6 possibilities that the code could be but you are only looking for 1 of those. 

Now if you can use BBB & AAA, it would be 1/8.