We have to answer the following question for an exercise:
„You have an infinite supply of letters from A-Z. With every draw you pick a random letter (p=1/26) from the supply. How many draws do you need to be 90% certain that you have every letter at least once? Show the calculations.“
I know that the probability for the first draw to be a new letter is 26/26 and so on (27-n)/26 for the n-th letter. I‘m not sure how I‘m supposed to add all those probabilities together for the „final“ probability to reach 90%. Can anyone give me a hint for it?