You do not need to be a human calculator to benefit from mental math. A handful of simple tricks can help you estimate totals, split bills, and make quick decisions with confidence.
To calculate a 15% tip, multiply the bill by 10% (move the decimal left) and add half of that. For a bill: .80 + .40 = .20. For 20%, just double the 10% figure.
To add large numbers quickly, round them up, add, and subtract the difference. 397 + 285 = (400 + 285) – 3 = 685 – 3 = 682.
Multiplying by 11 is simple: for 32 x 11, split the digits (3 and 2) and put their sum in the middle: 352. If the sum exceeds 9, carry the one: 57 x 11 = 627.
Practice these tricks for five minutes a day and you will be amazed how quickly they become second nature.