You should read up more on permutations and combinations and learn from examples. That was what I did, and still doing, cos I find myself weak in permutations too...
For e.g this website has some good examples:-
https://www.questionbank.4gmat.com/mba_p ... obability/
For your qn, 4 digit code can be visualized like 4 spaces, i.e _ _ _ _, which can be filled up by any of 26 letters.
At the first space, you can choose any of the 26 letters, right? And next, you go to the 2nd space, you find that you can choose any of the 26 letters too. So on..
That would mean we have combinations of 26*26*26*26 = 26^4. The same logic can be applied to the 5 digits to get 26^5. The qn is kind enough to let us re-use the letters so we do not have to factor anything else.
And since it says either 4 digit or 5 digit, it means that we need to add up the numbers cos we can choose from ANY of the combinations, be it 4 or 5 digit.
Hope this helps you...Maybe the experts could give their valuable inputs!
