To increase or decrease an amount by a percentage, do it in two steps. First find that percentage of the amount — the change. Then combine it: add for an increase, subtract for a decrease. To increase 60 by 10%: 10% of 60 = 6, then 60 + 6 = 66. To decrease 60 by 10%: 60 − 6 = 54. The change on its own is not the answer — you must apply it to the starting amount, and check the direction word 'increase' or 'decrease' before you add or subtract.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- increase
- make bigger — the new amount is more
- decrease
- make smaller — the new amount is less
- new amount
- the starting amount after the change is applied
Worked examples
Increase 60 by 10%
- 1.10% of 60 = 6.
- 2.60 + 6 = 66.
= 66
Increase 80 by 25%
- 1.25% of 80 = 20.
- 2.80 + 20 = 100.
= 100
Decrease 50 by 20%
- 1.20% of 50 = 10.
- 2.50 − 10 = 40.
= 40
For parents
Method: Two steps: the change, then add or subtract it.
What to say: Underline 'increase' or 'decrease' first so the +/− direction is decided before any arithmetic.
Common mistakes:
- Giving the change, not the new total (increase 60 by 10% → 6).
- Applying it the wrong way (increase 60 by 10% → 54).
- Adding the percent number itself (60 + 10 → 70).