Slightly more general formulation is THIS:


    How many distinguishable arrangements ("words") can be formed using two letters "a" and "b".


    The answer is:   =  = 10.


     So you need to take 5! = 120 of all possible permutations of 5 symbols and then divide it by 2! = 1*2  and  by 3! = 1*2*3  

     to account for repeating symbols.