MEPX
Chapter 5 of 11All chapters
  1. 1Why bother
  2. 2Splitting numbers
  3. 3Rounding and compensating
  4. 4Doubling and halving
  5. 5Shortcuts worth knowing
  6. 6Percentages
  7. 7Fractions and decimals
  8. 8Squares and roots
  9. 9Division
  10. 10Estimation and checks
  11. 11Building speed

Chapter 5 of 11

Shortcuts worth knowing

Nine, eleven, five and twenty five.

Special multipliers

To multiply by 9, multiply by 10 and subtract the number. To multiply by 5, halve the number and multiply by 10. To multiply by 25, multiply by 100 and divide by 4.

  • By 11 for a two digit number: add the digits and put the sum in the middle, so 34 times 11 is 374.
  • Carry when that middle sum passes 9: 87 times 11 is 957.

Squares near a round number

For a number ending in 5, multiply the leading part by the next number up and append 25. So 35 squared is 3 times 4 with 25 on the end, giving 1225.