Mathematics: Combinations and Counting

Question

You have 7 scrabble tiles A, B, C, D, E, F, G. Using these 7 tiles, how many different strings of 4 letters can you form?

Answer 1

Order is important: ABCD is considered a DIFFERENT order than DCBA

(7 pick 4) = 7! / (7-4)! = 7! / 3! = 7*6*5*4 = 42*20 = 840