McGraw-Hill Education GRE 2019

Probability with “and”: Multiply Them

If a question concerns the probability that an event occur multiple times, multiply

the probabilities of each event.

If a fair-sided coin is flipped three times, what is the probability the coin will

land on heads all three times?

A 0.125

B 0.25

C 0.5

D 0.75

E 1.5

SOLUTION: On any given flip, the probability that the coin will land on heads

is 0.5. Since the question concerns the probability that the coin will land on

heads all three times it is flipped, you should multiply these probabilities. 0.5

× 0.5 × 0.5 = 0.125. The correct answer is A.

Counting

Fundamental Counting Principle

To calculate the number of ways to combine groups, calculate the product of the

number of elements in each group.

Bob’s wardrobe contains 5 shirts, 7 pairs of pants, and 4 ties. If he wants to

select an outfit that consists of 1 shirt, 1 pair of pants, and 1 tie, how many

different outfits can he select?

A 16

B 20

C 28

D 35

E 14 0

SOLUTION: Since the question concerns the combination of different groups,

find the product of the number of elements in each group. 5 × 7 × 4 = 140.

The correct answer is E.

CHAPTER 12 ■ FROM WORDS TO ALGEBRA 357

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