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.
A worked example
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.
Try it yourself
512 − 99 =
63 + 49 =
Watch out
You adjust in the wrong direction: for 73 − 29 you do 43 − 1 = 42.
You took away too much, so you must give some back. When you add, it is the other way round.
You forget to adjust: for 46 + 19 you say 66.
After using the round number there is always one more step: adjust for the difference.
Tips
- It also works with 18, 28 and 98: then you are 2 out.
- For 99 the round number is 100, for 199 it is 200.
Why it works
19 is nearly 20, and 20 is much easier to work with. You use 1 too many, so you adjust for that afterwards. When you add, you take it away again; when you subtract, you add it back.