PSAT Math Multiple-Choice Question 518: Answer and Explanation

Question: 518

A group of students observed that there was a relationship between the hours a petri dish was left to incubate and the resulting number of cells in the petri dish. The graph below displays their findings as well as a best-fit line drawn by one of the students.

Which of the following is the correct approximate average hourly increase in the number of cells, based on the best-fit line in the graph?

• A. 0.06
• B. 0.43
• C. 6.00
• D. 92.00

Correct Answer: C

Explanation:

C

The question asks for the average hourly increase in the number of cells. When a question asks for a rate of change, it is asking about the slope of the graph. For slope questions, use the equation . Find two points on the best-fit line and plug them into the slope equation. Use the points (0, 50) and (5, 80), where y₂ = 80, y₁ = 50, x₂ = 5, and x₁ = 0. The slope is . The correct answer is (C).