Math practice · reimagined
Simple Interest Game
In this simple interest fight, an amount from 100 to 600 earns 2%, 4%, 5% or 10% a year for up to five years, and you type only the interest. Your rival sees the same numbers, and the first right answer becomes your fighter’s punch.
Ages 4–99+ · no download · plays in your browser · 23 languages
What is simple interest?
Simple interest is the extra that savings earn, or that a loan costs, worked out every year from the starting amount only. So each year adds the same extra. Lend a friend 10 marbles and ask for 1 extra marble a year. After 3 years they owe you 13 marbles: the 10 back, plus 3.
Every level of Interest
Example: 200 at 4% a year for 5 years
- The screen shows P=200, r=4% and t=5: an amount of 200, earning 4% a year, for 5 years.
- Multiply the amount, the rate and the years: 200 × 4 × 5 = 4000.
- The rate was 4 out of every 100, so divide 4000 by 100 to get 40.
- The savings earn 40 in interest, and 40 is the number you type.
Learn it step by step
Interest is what money earns for being lent out — or costs for being borrowed. Simple interest is the straightforward version: the same amount is earned every single year, because it is always worked out on the ORIGINAL sum.
Three things decide it. The principal P is what you started with. The rate r is the percentage earned per year. The time t is how many years it sits there. Multiply all three: I = P × r% × t.
Put £400 in at 5% for 3 years. Five percent of 400 is 20, so it earns £20 each year, and over three years that is £60. That is the interest — the account now holds £460, but the question asked what it EARNED, not what it holds. Read carefully which one is wanted.
Real bank accounts usually pay compound interest, where each year's earnings start earning too, so the total creeps ahead of this. Simple interest is the honest first model, and it is still exactly how many loans are quoted.
Find one year's interest, multiply by the number of years, and type the total interest earned.
Tips and common mistakes
- Start from 10%, which drops the last zero: 400 earns 40 a year. Halve it for 5%, giving 20, and take a fifth for 2%, giving 8.
- An easy slip is giving one year’s interest only. 500 at 2% earns 10 a year, so over 3 years it earns 30. Check t before you type.
- For interest calculation practice at home, play bank: 100 buttons saved at 5% a year. Ask what they earn after 1, 2 and 3 years: 5, 10 and 15.
How to play
Each round stacks three values on screen, such as P=600, r=10%, t=1 yr, and asks for the interest. Type only that on the keypad, never the new total; it is always a whole number from 2 to 300. Answer before your rival and Garlic attacks; the less of the round’s time you use, the harder it lands.
— Quick answers —
Interest: the short FAQ
- What does principal mean in simple interest?
- The principal is the starting amount, the sum that is saved or borrowed before any interest is added. In I = P × r% × t it is the P. Each year’s interest is worked out from it, and it stays fixed for the whole time. A loan of 2000 at 5% a year has a principal of 2000 and costs 100 in interest every year.
- How do you find the total amount after simple interest?
- Work out the interest, then add it to the principal. Savings of 1500 at 4% a year earn 60 each year, because 4% of 1500 is 4 × 15. After 3 years that makes 180 in interest, so the savings hold 1500 + 180 = 1680. In a fight you type the 180, since the question asks only for the interest.
- What is the fastest way to calculate simple interest?
- The fastest way is to multiply the rate by the years first, which gives the whole percentage in one go. At 4% for 5 years, savings earn 20% in total, and 20% of 300 is 60. This works because simple interest adds the same percentage every year. It also shows that 5% for 4 years earns exactly as much.
- Why is my child’s simple interest answer 100 times too big?
- Most often the percent sign was skipped. 300 × 5 × 2 makes 3000, but a rate of 5% means 5 out of every 100, so the product still has to be divided by 100, giving 30. A quick sense check catches it: at 10% a year or less, five years can never earn more than half the amount.
- Does it cost anything to use this financial math game with a class?
- No, the simple interest level is free, opens in any browser and needs no pupil accounts or downloads. For simple interest practice at the board, draw a table for savings of 300 at 4% and let pupils fill in years 1 to 5: 12, 24, 36, 48 and 60. The equal steps show what makes the interest simple. Then pupils fight on tablets, and the schools plan, priced per student, adds a dashboard of who is improving.