Forty-three percent of people have type O blood, 15 percent have the Rh negative factor and 52 percent have one or the other. What share have both?

1. The addition rule says the union equals the sum of the parts less the overlap. 2. So 0.52 equals 0.43 plus 0.15 minus the joint probability. 3. The sum of the two parts is 0.58. 4. The joint probability is therefore 0.58 minus 0.52, which is 0.06. 5. So six percent of people have both type O blood and the Rh negative factor. 6. Answering nine percent subtracts in the wrong direction and gives a joint probability larger than it should be. _Source: OpenStax Introductory Business Statistics (CC BY 4.0), Ch 3 "Probability Topics", section 3.3 Two Basic Rules of Probability_