The Stock Market

10 min

Teacher Prep
Setup
Remind students of sixth grade strategies for answering the percentages questions.

Narrative

In this Warm-up, students recall previous course work solving problems involving percentage increase or decrease. This thinking will be useful when students use percentages to determine the change in stock prices in following activities. 

Launch

Give students 4 minutes of quiet work time, and follow with a whole-class discussion. If necessary, remind students that they can use any strategies from previous courses to answer the percentages questions.

Student Task

  1. Lin deposited $300 in a savings account that has a 2% interest rate per year. How much is in her account after 1 year? After 2 years?
  2. Diego wants to sell his bicycle. It cost $150 when he bought it but has depreciated by 15%. How much should he sell it for?

Sample Response

  1. After one year: $306, because 300⋅1.02=306300 \boldcdot 1.02 = 306. After two years: $312.12, because 306⋅1.02=312.12306 \boldcdot 1.02 = 312.12.
  2. $127.50, because 150⋅0.85=127.5150 \boldcdot 0.85 = 127.5
Activity Synthesis (Teacher Notes)

The goal of this discussion is to review methods for calculating a percent increase or decrease. Invite students to share their responses and reasoning to each problem. Some strategies students may use include:

  • Calculating the magnitude of the change and adding or subtracting it from the initial amount.
  • Reasoning that a 2% increase is equivalent to multiplying the initial amount by 1.02 and that a 15% depreciation is equivalent to multiplying the initial amount by 0.85. 
  • Writing and solving an equation in the form A% of B=CA\%\text{ of }B=C.
Anticipated Misconceptions

If students forget about using an equation to calculate percent increase or decrease, remind them that they can use the equation y=kxy=kx to figure out the answers to the questions.

Standards
Addressing
  • 7.RP.3·Use proportional relationships to solve multistep ratio and percent problems.
  • 7.RP.A.3·Use proportional relationships to solve multistep ratio and percent problems. <span>Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.</span>

10 min

15 min