Student: A card shop contains 10 birthday cards, 10 holiday cards, and 9 graduation cards. If three cards are purchased at random from the shop, what is the probability that the three cards will be of the same type?
Options:
(A) 3/29
(B) 12/58
(C) 12/840
(D) 7/841
(E) 12/841.
Another student: Let's say, hmmm...
It doesn't matter which type of card you choose, so:
Probability of choosing 1st card: 1
Probability of choosing 2nd card of the same type: 4/29
Probability of choosing 3rd card of the same type: 3/29
Multiply and you get 12/841 Final answer: (E).