Official Guide Explanation:
Data Sufficiency #61

Background

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Solution and Metadata

Question: 61
Page: 278
Difficulty: 5 (Moderate)
Category 1: Arithmetic > Ratio and Proportion >
Category 2: Word Problems > Rate Problems > other

Explanation: Statement (1) is insufficient: if you want a number of guests, a percentage of guests will not be sufficient. (Unless you can combine it with a number in the question, but there are none in this example.) Statement (2) is also insufficient: 120 scoops could be 10 single scoops and 55 double scoops, or 100 single scoops and 10 double scoops (or any number of other possibilities).

Taken together, the statements are sufficient. (2) implies an equation: x + 2y = 120, where x is the number of people who get one scoop, and y is the number who get two scoops. (1) also gives you an equation: (0.6)y = x + y. Together, that's two linear equations with two variables. You can solve for both variables; once you know y, you can answer the question, so (C) is the correct choice.

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