Vanishing sums and almost-vanishing
You learn that if you take away everything, 0 is left, and if you take away almost everything, 1 is left.
How to work it out
- Do you take away the same number? Then everything is gone and you have 0 left.
- Do you take away one less? Then exactly 1 is left.
A worked example
15 − 15 = ?
- Take away everything.
- 15 − 15 = 0.
Another example
23 − 22 = ?
- Take away almost everything (one less than 23).
- 23 − 22 = 1.
Try it yourself
15 − 15 =
10 − 10 =
Watch out
You say 8 − 8 = 8.
If you take away everything, nothing is left. 8 − 8 = 0.
Tips
- Anything minus itself is 0. Anything minus one less than itself is 1.
Why it works
If you have 8 and take 8 away, you have 0. If you have 8 and take 7 away, you have 1. This helps you check subtraction: the answer must be smaller than what you start with.