SOLUTION: 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.?

7*6*5*4 = 4220 different words.


You have 7 option to select first letter;

    then 6 options to select second letter

    . . .   and so on  . . . 


So your answer is the product of 4 integer factors that are consecutive integer numbers in descending order, starting from 7.