Calculating with 25 and 50
You learn to work easily with 25 and 50 by thinking in quarters and halves.
How to work it out
- Think of money: 25 is a quarter, 50 is a half, and four quarters are 100.
- If you go over the hundred, jump there first.
- Then work out the rest in quarters.
A worked example
175 + 50
- First jump to the next hundred: 175 + 25 = 200.
- 50 is two quarters, so there is still 25 to add.
- Add the rest: 200 + 25 = 225.
- So 175 + 50 = 225.
More examples
450 − 75
- First jump back to the hundred: 450 − 50 = 400.
- Take away the rest: 400 − 25 = 375.
325 + 75
- Leave the 300 as it is.
- Add 25 + 75 = 100.
- Put them together: 300 + 100 = 400.
Try it yourself
325 + 75 =
500 − 125 =
Watch out
You think 75 + 25 is 90 or 110.
Think in quarters: three quarters and one quarter make four quarters, and that is exactly 100.
Tips
- Think of money: 25 pence is a quarter of a pound, and four lots of 25 pence make 100 pence.
- Two halves and four quarters both make exactly 100.
Why it works
25 is a quarter of 100, and 50 is half of it. Four quarters or two halves make exactly 100. If you know that, you can see straight away that 175 + 25 makes 200.