Quick methods with 9, 19, 29
You learn to add and subtract numbers like 19, 29 and 99 by using the round number first.
How to work it out
- Make the second number a round number: 19 becomes 20.
- Calculate with the round number.
- You added or took away too much: adjust that difference.
46 + 19
- Make 19 into 20.
- Work it out: 46 + 20 = 66.
- You added 1 too many, so take it away: 66 − 1 = 65.
More examples
73 − 29
- Make 29 into 30.
- Work it out: 73 − 30 = 43.
- You took away 1 too many, so add it back: 43 + 1 = 44.
256 + 99
- Make 99 into 100.
- Work it out: 256 + 100 = 356.
- You added 1 too many, so take it away: 356 − 1 = 355.
58 + 18
- Make 18 into 20.
- Work it out: 58 + 20 = 78.
- You added 2 too many, so take them away: 78 − 2 = 76.