There are a total of 7 + 9 + 2 = 18 balls in the vase, therefore, when not replacing the first ball, there would be 18 · 17 = 306 ways to pull 2 balls. There are 2 ways to choose the first black ball and then 1 way to choose the second black ball so there are 2 · 1 = 2 ways to choose 2 black balls. Therefore, the probability is 2 / 306 = .
0 is considered a whole number, but it is not a natural number. The natural numbers start at +1 and go forward. ) is not positive or negative, but being that it is a whole number, it is also a rational number.
