How to use the addition rule?

Question

All human blood can be types as one of O, A, B or AB. Distributions of the types however, varies a bit by race. In a randomly chosen sample of African Americans, here is the distribution: 

Type O - 0.49

Type A - 0.27

Type B - 0.20

Type AB - ___

Using the information above, answer the following questions:

1. What is the probability of Type AB blood? Explain why.

2. What is the probability that the person chosen does not have type AB blood? 

3. Maria has type B blood. She can safely receive blood transfusions from people with blood types O and B. What is the probability that a randomly chosen African American can donate blood to Maria? 

Answer 1

Probability of Type AB = 1 - 0.49 - 0.27 - 0.20 = 0.04 (sum of probabilities is 1)

Probability not type AB = 1 - 0.04 = 0.96

Probability Type B or Type O = 0.49 + 0.20 = 0.69