Cracking the SI Formula
Use inverse operations to find Principal, Rate, or Time from the Simple Interest formula.
Equation
y = x * (5/100) * 1
Graph
Table
| x | y |
|---|---|
| 5000 | 250 |
| 7500 | 375 |
| 10000 | 500 |
| 12500 | 625 |
| 15000 | 750 |
| 17500 | 875 |
| 20000 | 1000 |
What this lesson covers
What you do
You shape the function y = x * (r/100) * t and watch the graph answer.
Challenges to clear
- Verify the calculation: With r=10 and t=3, does a Principal of 15000 yield an Interest of 4500? Check the table row for x=15000.
- Confirm the relationship: If the Principal is 10000, the interest should be 3000 (10% of 10000 × 3 years).
- A longer loan: slide the time t to 4 years.
- Now the rate: a principal of 20000 must earn 4000 over those 4 years. Simple interest never compounds — each year earns the same.
Check yourself
If SI = 4500, R = 10%, and T = 3 years, which algebraic step correctly isolates the Principal P?
Why do we divide the Simple Interest by the product of Rate and Time (R*T/100) to find the Principal?
- P = 4500 / (10 * 3)
- P = 4500 / (0.10 * 3) — correct
- P = 4500 * 0.10 * 3
- P = 4500 / 0.10 / 3
- Because Principal is the result of multiplying Interest by Rate and Time.
- Because Interest is calculated as P * (R*T/100), so we reverse the multiplication by dividing. — correct
- Because Rate and Time are subtracted from the Principal to get Interest.
- Because the formula requires adding Rate and Time before dividing.
Think about it
- We need the Principal (x) that produces Rs 4500 interest at r = 10% over t = 3 years. Which table row will show 4500?