Hello :) This is a great task for combinatorics. Lets look at it this way: How many letters can you put in the first place of possible password? 7 - you have 7 letters in total you can put as first. Now, how many letters can you put as second? 6. Why? Because we have already used one letter and we cannot repeat it. It works the same with third place (5) and fourth place (4). Now, we can use one interesting property - combinatorics rule of multiplication, which tells us 7 times 6 times 5 times 4 is equal to all possibilities = 840. You can image it this way. To each of 7 letters at the first place, you can put another 6 - this leaves us with 42 combinations and so on ;). All in all, we can form 840 different passwords.