Actual, it is put 1a,2b,3C into 6 slot, but the same word can not be counted more than twice. In fact, after 1a and 2b are in the slot, no matter how you put remaining 3c, it will be the same word. So the problem become how many different way you can put 1a and 2b into 6slot. Thus, we put 1a first, there are 6 possibilitys. After a is positoned, there left 5 slot. Among these 5, if you put 1b at the position 1, there are 4 way to put second b. Then put 1b in the 2nd postion, there are 3 way to put second b. So on and on. there are total 10 way to put there 2b into remaining 5 positions. Therefore, the total number of posibility would be 6*10=60.Correct or not.
