MEPX
Chapter 10 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 10 of 11

Estimation and checks

Getting close on purpose.

Front end estimation

Round every number to one significant figure and do the sum. 312 times 48 becomes 300 times 50, giving about 15,000, which is close enough to catch a wrong answer.

  • Round one number up and the other down to keep the error small.
  • Always sanity check the size of the answer before the digits.

Checking with digit sums

Casting out nines catches many arithmetic slips: the digit sum of the answer should match the digit sums of the inputs combined the same way. It will not catch a swapped pair of digits.