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Teaching video
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Grade 7 chapters and video availability01 · Read and understand
What you will learn
- Explain how to match time units in simple interest.
- Solve the two practice problems and explain how the assumptions affect the answers.
Before you start
Percentage multiplication and month-to-year conversion.
Keep paper nearby. Read the question once for the context, then again to identify what is known and what you need to find.
Start with a question
For arithmetic practice, a fictional $600 balance earns 4% simple annual interest for 18 months. Find the interest and final balance.
Why this math matters
In a simple-interest model, interest is proportional to principal, annual rate, and elapsed years. Treat this as a mathematical model; identify compounding, fees, and time conventions before modeling a real agreement.
Set up the model
A useful answer starts with clear assumptions:
- This example uses simple interest with no compounding or fees.
- One year is 12 months and the annual rate stays fixed.
02 · Work through the example
Follow the reasoning, one step at a time.
Try to predict the next step before reading it. After each calculation, explain why the operation makes sense and how it helps answer the original question.
Work through it with Amar
See this example unfold.
The complete worked example, one idea at a time.
Match time units in simple interest
PausedQuestion: Start with the question. Paused.
Question
Start with the question
For arithmetic practice, a fictional $600 balance earns 4% simple annual interest for 18 months. Find the interest and final balance.
Before you calculate
Read what is known and what you need to find. Make a prediction before moving to the first calculation.
Starts paused. Play advances through the full text at a reading pace; pause whenever you need more time. Previous, Next, and the phase buttons let you set your own pace. Playback pauses when this walkthrough leaves the screen or you switch tabs.
Your device’s reduced-motion setting keeps each phase still. Manual controls remain available. The full written solution stays below.
Represent the quantities
P = 600; r = 0.04; t = 18/12 = 1.5 years
Match the time unit to the annual rate before using the formula.
Apply the relationship
I = Prt = 600(0.04)(1.5) = $36
Only the original principal earns interest in this stated model.
Check and interpret
final balance = 600 + 36 = $636
Interest is the increase; the final balance also includes the principal.
The result
final balance = 600 + 36 = $636
Interest is the increase; the final balance also includes the principal.
Common mistakes to catch
- Using 18 as the time with an annual rate counts 18 years.
- Simple interest does not repeatedly earn interest on previous interest.
03 · Practice independently
Try it before revealing the answer.
Use paper or a calculator as needed. Write your units and reasoning, then open the hint or explanation to check your approach.
Practice 1
In the same simple-interest model, what interest does $250 earn at 6% per year for 8 months?
Show a hint
Convert 8 months to 2/3 year.
Reveal answer and explanation
$10
250(0.06)(2/3) = 10.
Practice 2
A fictional $400 principal earns $48 simple interest over 3 years. Find the annual rate.
Show a hint
Rearrange I = Prt to r = I/(Pt).
Reveal answer and explanation
4% per year
48/(400 × 3) = 0.04.
Take the idea with you
Treat this as a mathematical model; identify compounding, fees, and time conventions before modeling a real agreement.
04 · Reflect and continue
Can you explain it in your own words?
Before moving on, explain the main idea without looking at the worked example. Try both practice questions, check your reasoning, and name one mistake you now know how to avoid. Return to a step if you still need support.
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