statistics
posted by Emily .
An Urn contains 7 red and 3 white balls. Two balls are drawn in succession without replacement. If the second ball is white, what is the probability that the first ball was white?

It doesn't matter what the second ball is. Since no balls have been drawn before the first ball, probability = 3/10.

A box contain 3 red balls and 6 white balls. One ball was pulled and was seen its collar, and after that was pulled another ball.Find the probability that the first ball was white and the second was red ?