Chapter 3 of 11All chapters
Chapter 3 of 11
Rounding and compensating
Move to a round number, then correct.
Round then adjust
999 plus 111 is easier as 1000 plus 111 minus 1, which gives 1110. Round one number to something friendly, do the easy sum, then undo the rounding.
- For subtraction, 1000 minus 487 is easier counting up: 13 to 500, then 500 more.
- Always note which direction you rounded, since that decides whether you add or subtract at the end.
Multiplying near a round number
98 times 7 is 100 times 7 minus 2 times 7, so 700 minus 14, which is 686. The further from round the number is, the less this helps.