Page 58 - Probability Demystified

CHAPTER 3 The Addition Rules 47

2. a. P(Buick or Chevrolet) ¼ P(Buick) þ P(Chevrolet)

4 6 10 1
¼ þ ¼ ¼
20 20 20 2

b. P(Chrysler or Chevrolet) ¼ P(Chrysler) þ P(Chevrolet)
3 6 9
¼ þ ¼
20 20 20

15 10 25
3. a. P(N or G) ¼ P(N) þ P(G) ¼ þ ¼
53 53 53

23 5 28
b. P(HO or O) ¼ P(HO) þ P(O) ¼ þ ¼
53 53 53

6 4 10 1
4. a. P(white or green) ¼ P(white) þ P(green) ¼ þ ¼ ¼
20 20 20 2

2 8 10 1
b. P(blue or red) ¼ P(blue) þ P(red) ¼ þ ¼ ¼
20 20 20 2

10 6 16
5. a. P(history or math) ¼ P(history) þ P(math) ¼ þ ¼
25 25 25

5 4 9
b. P(reading or science) ¼ P(reading) þ P(science) ¼ þ ¼
25 25 25

Addition Rule II

When two events are not mutually exclusive, you need to add the probabilities
of each of the two events and subtract the probability of the outcomes that
are common to both events. In this case, addition rule II can be used.

Addition Rule II: If A and B are two events that are not mutually exclusive,
then PðA or BÞ¼ PðAÞþ PðBÞ PðA and BÞ, where A and B means the num-
ber of outcomes that event A and event B have in common.