A percentage is simply a number expressed as a fraction of 100. Knowing a few mental math tricks makes percentage calculations much faster.

Key Tricks

  • 10% rule: move the decimal point one place left (10% of 350 = 35)
  • 5%: halve your 10% result (5% of 350 = 17.5)
  • 15% tip: find 10%, then add half of that (10% + 5%)
  • Percentage of a number: multiply by the decimal (25% of 80 = 0.25 × 80 = 20)

Common Uses

Calculating VAT (20%): multiply the price by 1.2. Finding a discount: multiply the original price by (1 − discount fraction). For example, 30% off £60 = 60 × 0.7 = £42.