You go to the bank and open an account with 4% interest. How long will it take for your money to double?
interest rates and doubling your money
You want to buy the latest gizmo, one shop is selling it as 23% off of £50 and the other as 8% off of £40. Who is cheapest?
Rob Eastaway's book - Maths on the Back of an Envelope: Clever Ways to (Roughly) Calculate Anything