A marble is chosen at random. If you're told the marble is red, what is the probability that it has the number 7 on it?

Question

A jar contains 10 red marbles, numbered 1 to 10, and 12 blue marbles numbered 1 to 12.  

a) A marble is chosen at random. If you're told the marble is red, what is the probability that it has the number 7 on it

b) The first marble is replaced, and another marble is chosen at random. If you're told the marble has the number 1 on it, what is the probability the marble is red?

Answer 1

Probability at its most basic is the ratio of how many times the desired outcome can happen out of the total number of outcomes. P = desired/total

a) the only desired outcome is a number 7, but we know that the marble HAS to be red. Therefore, there are only 10 possible outcomes, and one desired outcome, so P =1/10 or 10%

b) If you are told the marble has a number one on it, there are only two marbles that it could possibly be; the red and the blue. therefore: P =1/2 or 50%