In a study of blood donors, 200 were classified as group o and 250 had a classification other than group o. what is the probability that a person will have group o blood? (enter your answer as a fraction.)

Probability of O = (number with O)/(Total number of cases)

It always works like this.

P(O) = 200/450 = 4/9

If you want to cheat, paste the formula in Wolfram Alpha. However, this one is not that difficult. From the fact that x and y are squared you can infer that this is a circle. The 9 reveals that the radius r^2 is 9, so radius is 3.

The (x-2) tells us that the circle is shifted to the right by 2. Likewise, y+1 shifts it down by one. So the center is at (2,-1). If the equation would be x^2 + y^2 = 9, you'd have a circle exactly on the origin.

Hopefully this helps you break down the equation and pick the right picture!