LSAT Explanation PT 34, S3, Q21: Thirty years ago, the percentage of

LSAT Question Stem

Which one of the following, if assumed, helps most to justify the conclusion drawn above? 

Logical Reasoning Question Type

This is a Sufficient Assumption question. 

Correct Answer

The correct answer to this question is C. 

LSAT Question Complete Explanation

Let's first analyze the argument in the passage. The passage states that thirty years ago, single persons spent twice the percentage of their income on food than they do today. It also mentions that incomes have risen over the past thirty years. Based on this information, the passage concludes that incomes have risen at a greater rate than the price of food in that period. This conclusion is based on the premise that the percentage of income spent on food has decreased.

To make this argument more understandable, let's use a simple example. Suppose that thirty years ago, a single person spent 20% of their income on food, and today, they spend only 10% of their income on food. If we know that their income has increased during this time, we can conclude that their income has risen at a faster rate than the price of food.

An "Evaluate" question for this argument could be: "Has the quantity and type of food purchased by single persons remained the same over the past thirty years?"

Now, let's discuss the answer choices for the question, which is a Sufficient Assumption question. We are asked to identify which assumption, if true, helps most to justify the conclusion drawn above.

a) The amount of food eaten per capita today is identical to the amount of food eaten per capita thirty years ago.

This answer choice is not sufficient to justify the conclusion because it focuses on the amount of food eaten per capita, which includes both single and married persons. We need information specifically about single persons to justify the conclusion.

b) In general, single persons today eat healthier foods and eat less than their counterparts of thirty years ago.

This answer choice weakens the argument by providing an alternative explanation for the decrease in the percentage of income spent on food. If single persons today eat less, it could be the reason for the decreased percentage, rather than the rate of income increase.

c) Single persons today, on average, purchase the same kinds of food items in the same quantities as they did thirty years ago.

This answer choice is correct because it eliminates the possibility that the decrease in the percentage of income spent on food is due to changes in the quantity or type of food purchased by single persons. By assuming that the same kinds of food items and quantities are being purchased, we can conclude that the decrease in the percentage is due to incomes rising at a greater rate than the price of food.

d) The prices of nonfood items single persons purchase have risen faster than the price of food over the past thirty years.

This answer choice does not help justify the conclusion because it focuses on the prices of nonfood items, which are not relevant to the argument about the relationship between income and the price of food.

e) Unlike single persons, families today spend about the same percentage of their income on food as they did thirty years ago.

This answer choice weakens the argument by suggesting that the observed decrease in the percentage of income spent on food for single persons might not be indicative of a general trend. If families have not experienced a similar decrease, it raises questions about the validity of the conclusion.

In summary, the correct answer is C because it eliminates the possibility that changes in the quantity or type of food purchased by single persons are responsible for the decrease in the percentage of income spent on food, thus strengthening the conclusion that incomes have risen at a greater rate than the price of food.