five coins whose faces are marked 3 and 4 are thrown.the chance of obtaining a total of sum 18 is

There is only one way to obtain 18 i.e. 2 3`s and 3 4`s . Total number of possible outcomes is 2^5 since there are 5 places of two possibilities each . Probability of sum 18 is (5C3)/(2^5).5C3 implies choose 3 of 5=5!/(3!*2!)Since we have to get 4 on 3 out of 5 coins .