Bunuel wrote:
A box contains red and blue balls only. If there are 8 balls in total, how many red balls are in the box?
(1) If two balls are randomly selected without replacement, the probability that both balls are red is 5/14
(2) If two balls are randomly selected without replacement, the probability is 15/56 that the first ball is red and the second ball is blue.
Kudos for a correct solution.
For this type of question we should only know the principles of probability and don't need to make any calculations to find the correct answer:
We know that this box contain only red and blue balls (
important: this approach will be work only for two types of items).
So if we have information about probability randomly selected balls of
ONE color (any quantity) we can find how much balls of each color in box.
1) in this statement we have information about probability of two balls the
SAME color - this is
sufficient and we don't need any calculations, because this is DS task and we should save time
If we have probability for balls with
DIFFERENT colors we can't find how much balls of each color in box
2) in this statement we have information about probability of two balls the same color - this is
insufficient and we don't need any calculations, because this is DS task and we should save time
So answer is A
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