Outcomes - SSAT Upper Level Quantitative

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Question

A fair six-sided die has on its faces the numbers 1, 1, 1, 2, 2, 3; a fair eight-sided die has on its faces the numbers 1, 1, 1, 1, 2, 2, 3, 4. If both dice are rolled, what is the probability that the numbers add up to 3 or less?

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Answer

In order for the roll to add up to 3 or less, neither die can come up 3 or 4. Therefore, we look at the probabilities that each die can come up 1 and 2.

The six-sided die comes up 1 with probability ; it comes up 2 with probability .

The six-sided die comes up 1 with probability ; it comes up 2 with probability .

A roll of 3 or less can happen three ways:

Case 1: Both dice come up 1. The probability of this, by the Multiplication Principle, is

Case 2: The six-sided die comes up 1 and the eight-sided die comes up 2. The probability of this, by the Multiplication Principle, is

Case 3: The six-sided die comes up 2 and the eight-sided die comes up 1. The probability of this, by the Multiplication Principle, is

These are mutually exclusive events, so add these probabilities:

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