Question

a deck of cards has only red cards and black cards. the probability of a randomly chosen card being red is 1 3 . when 4 black cards are added to the deck, the probability of choosing red becomes 1 4 . how many cards were in the deck originally?

Answer 1

Answer: The answer is (264)×(261)(525), because (264)×(261) are the possibilities for choosing 4 red cards and 1 black card from a deck of 52 cards, while (525) are the possibilities for choosing 5 cards from a deck of 52 cards.

So, your first solution is almost correct. On why your second solution is wrong, you'll have to explain your thought process.

Explanation: