# Lesson 15

Finding This Percent of That

### Lesson Narrative

Students have practiced solving three different types of percentage problems (corresponding to finding $$A$$, $$B$$, or $$C$$ respectively when $$A\%$$ of $$B$$ is $$C$$). This lesson focuses on finding “$$A\%$$ of $$B$$” as efficiently as possible. While the previous lesson used numbers that students could calculate mentally, the numbers in this lesson are purposefully chosen to be difficult for students to calculate mentally or to represent on a double number line diagram, so as to motivate them to find the simplest way to do the calculation by hand.

The third activity hints at work students will do in grade 7, namely finding a constant of proportionality and writing an equation to represent a proportional relationship.

### Learning Goals

Teacher Facing

• Choose and create diagrams to calculate A% of B, and explain (orally) the solution method.
• Generalize a process for finding A% of B and justify (orally) why this can be abstracted as $\frac{A}{100} \boldcdot B$.
• Identify equivalent expressions that could be used to find A% of B and justify (orally) that they are equivalent.

### Student Facing

Let’s solve percentage problems like a pro.

### Student Facing

• I can solve different problems like “What is 40% of 60?” by dividing and multiplying.

Building On