Idea 17of 23Builds onGroup to make a friendly numberCount up to subtract
Use a friendly number, then fix it
How can 35 000 help you add 34 999?
Today you'll discover
- how to swap in a friendly number
- how much to fix
- which way to fix it
Think first34 999 + 12 345
You add 35 000 to 12 345 instead of 34 999. What must you do next?
Decide first, then press Next. Ask how far the friendly number is from the real one.
An open number line starting at 12 345. A long jump of +35 000 lands on 47 345; a short jump back of −1 lands on 47 344. The working reads 12 345 + 35 000 − 1 = 47 344. The line is not to scale. The last step asks for 26 500 + 9 998.
See the whole ideathe big idea and the key word, in one place
The big idea
Add a friendly number close to the real one, then fix the answer by the difference.
Easy to get wrongFix it the right way. Rounding 34 999 up to 35 000 made the sum 1 too big, so you take 1 away, not add it.
Key word
- compensation
- changing a number to a friendly one, then fixing the answer by the same amount
Ready to use it?
- Add or subtract using a friendly number, then fix the answer.