Dot problems with minus
You learn to find the missing number under the dot in a subtraction.
How to work it out
- If the dot is at the end, take the answer away from the start.
- If the dot is at the start, add the two numbers together.
A worked example
70 − · = 45
- The dot is at the end.
- Take the answer away from the starting number: 70 − 45 = 25.
- Check: 70 − 25 = 45.
- The number under the dot is 25.
More examples
· − 25 = 50
- The dot is at the start.
- Add the two numbers together: 50 + 25 = 75.
- The number under the dot is 75.
100 − · = 64
- The dot is at the end.
- Take 64 away from 100: 100 − 64 = 36.
- The number under the dot is 36.
Try it yourself
· − 36 = 64
· − 150 = 350
Watch out
You always subtract, even when the dot is at the start.
If the dot is at the start, the starting number was bigger than the answer. So add the numbers together.
Tips
- Dot at the end: subtract. Dot at the start: add.
Why it works
With a subtraction it depends on where the dot is. If the dot is at the end, you find how much was taken away with a subtraction. If the dot is at the start, you find the starting number by adding the other two together.