Probability that this stamp is one that you haven't seen before Suppose you have 4 stamps with probabilities 0.2, 0.4, 0.3, and 0.1, respectively. Each time you pick a stamp is independent of every other pick. Suppose you pick your 10th stamp. What is the probability this stamp is one you haven't seen before?
How should I approach these kinds of problems? Should I do 1-prob(I've seen this stamp before)?

Classify according to the card that is revealed:
What is the probability it is the first stamp?
the probability it hasn't appeared before is $0.2^9$ since the probability a stamp other than the first one appears in each pick each time is $0.8$ and this happens $9$ times. and the probability it is picked in the tenth pick is $0.2$ so the probability for the first stamp is $0.8^9\times 0.2$
Using this method the probabilities for each stamp are:
first stamp $0.8^9\times 0.2$
second stamp $0.6^9\times 0.4$
third stamp $0.7^9\times 0.3$
fourth stamp $0.9^9\times 0.1$
So the desired probability is the sum of these
