Money Math · Lesson Plan

Calculating Principal, Rate & Time

The same simple-interest formula (I = prt) can solve for any of its parts. This lesson reviews principal, rate, and time, then shows how to rearrange I = prt to find the missing value — how long it takes to earn a target amount, or what rate a loan charged. Students practice with ten money problems involving savings, loans, college debt, and comparing credit-card rates.

Grades 7–10 Lesson Plan 30–45 minutes Free Lesson Plan Lesson & Worksheet: Full Membership

Calculating principal, rate, and time lesson illustration

For Teachers

Lesson at a glance

Topic: Money Math
Grade Level: Grades 7–10
Resource Type: Lesson + Worksheet
Estimated Time: 30–45 minutes
Format: Lesson + worksheet + answer key
Materials: Printable lesson, worksheet, answer key, calculator

What Students Learn

Learning objectives

Identify principal, rate, time, and interest in a problem
Rearrange I = prt to solve for an unknown value
Solve for time needed to earn a target amount of interest
Solve for the rate charged on a loan
Compare interest rates to find the better deal

Materials

What you’ll need

Printed copies of the lesson and worksheet (one per student)
Calculators
Pencils
Whiteboard or projector for worked examples

Key Terms

Vocabulary

Principal: The amount of money borrowed or saved.
Rate: The percent paid or charged, often annual.
Time: How long the money is in use.
Interest: Money earned on savings or paid on borrowing.
Annual rate: An interest rate stated per year.
Solve for: Rearrange a formula to find an unknown quantity.

For Teachers

Lesson plan

Estimated time: one 30–45 minute class period.

Lesson sequence

Review (8 min). Recall principal, rate, time, and the formula I = prt.
Solve for the unknown (15 min). Work the examples: solve for time ($345 at 4% to earn $55.20 → 4 years) and for rate ($500 over 2 years owing $30 → 3%).
Worksheet (15 min). Students solve ten problems, finding interest, principal, rate, or time as needed and comparing two credit-card rates.
Review (7 min). Check answers with the key.

Assessment

Assess the completed worksheet against the included answer key.

Discussion

Discussion questions

How can one formula solve for principal, rate, time, or interest?
How would you rearrange I = prt to find the rate?
Why might you want to know how long it takes to reach a savings goal?
When comparing two credit-card rates, which is better and why?
How does a longer loan term change the total interest paid?

Printable Lesson, Worksheet & Answer Key