This Problem Set is a part of the Lesson 3, Unit 3, Grade 6. Students use positive and negative numbers to indicate a change (gain or loss) in elevation with a fixed reference point, temperature, and the balance in a bank account. Students choose an appropriate scale for the number line when given a set of positive and negative numbers to graph. A Look at Sea Level The purpose of this example is for students to understand how negative and positive numbers can be used to represent real-world situations involving elevation.