Finance and shopping percentage guide
Interest, tax and discounts are all the same arithmetic wearing different hats. Here is where the money quietly leaks.
Simple and compound interest
Simple interest: Principal × Rate × Years. $2,000 at 4% for 3 years earns $240.
Compound interest: Principal × (1 + Rate)^Years. The same $2,000 compounded annually earns $249.73.
Tax, inclusive and exclusive
A $120 price that includes 20% tax is $100 net plus $20 tax — not $96 plus $24.
To strip tax out, divide by 1 plus the rate. To add it, multiply by the same factor.
Why stacked discounts disappoint
50% off followed by 20% off leaves 0.5 × 0.8 = 0.4 of the original price, so the true saving is 60%, not 70%.
The order of two percentage discounts never changes the final price — but the order of a discount and a tax can.
Before a big purchase, calculate the final price yourself first. It takes ten seconds and settles any till-side surprises.
Put it to work