Simple interest
Calculate simple interest using I = Prt, where P is principal, r is annual rate, and t is time in years, with real-world financial applications in Grade 9 math.
Key Concepts
Property Simple interest is interest paid on the principal only. To find simple interest, use the formula $I = Prt$. Explanation Imagine you plant a money tree that only grows fruit on its original branches. That’s simple interest! It calculates earnings only on your starting amount (the principal). It’s a steady, predictable way to grow your money, adding the same fixed amount each period. It’s simple, straightforward, and easy to calculate, but it doesn't have that explosive growth power. Examples A 2000 dollars investment at $4\%$ simple interest for 5 years earns $I = 2000(0.04)(5) = 400$ dollars in interest. To find the total after 3 years on a 1000 dollars loan at $7\%$ simple interest: $I = 1000(0.07)(3) = 210$ dollars. The total owed is $1000 + 210 = 1210$ dollars. If you earned 600 dollars on a 4000 dollars investment over 3 years, the rate was $600 = 4000 \cdot r \cdot 3$, so $r = 0.05$ or $5\%$.
Common Questions
What is the simple interest formula?
Simple interest is calculated using I = Prt, where I is the interest earned, P is the principal (initial amount), r is the annual interest rate as a decimal, and t is the time in years. The total amount after interest is A = P + I = P(1 + rt).
How does simple interest differ from compound interest?
Simple interest is calculated only on the original principal amount, giving a fixed amount of interest each period. Compound interest is calculated on the principal plus previously earned interest, causing exponential growth. Simple interest produces a linear function while compound interest produces an exponential function.
What is a real-world example of simple interest?
A common example is a personal loan or savings certificate. If you deposit $1,000 at 5% annual simple interest for 3 years, you earn I = 1000 × 0.05 × 3 = $150, giving a total of $1,150. Simple interest is also used in some car loans and short-term bonds.