Permutation/Combination
You have a set of four different coloured flashlights that you use to send messages to your friend who lives across from you on the same street.
(For purposes of this question, a 'message' is considered to be a single sequence of one or more coloured lights, displayed one after the other.)
Find the number of different messages you can send using two different coloured flashlights displayed one after the other.

Please help me :(