A percentage decrease shows by what share a value has shrunk. You compute (old − new) ÷ old · 100, always relative to the old value. Worked example: from 250 to 200 that is a 20% decrease. Matches the Grade 7 curriculum.
A percentage decrease is the drop divided by the old value, times 100: (old − new) ÷ old · 100. From 250 to 200 that is (250 − 200) ÷ 250 · 100 = 20% decrease. To reduce a value by p%, subtract value · p ÷ 100.
At a glance
Summary of this tutorial
Example
250 → 200
Method
(old − new) ÷ old · 100
Steps
4
Result
20%
Check
250 · 0.8 = 200 ✓
Grade level
Grade 7 (ages 12–13)
Worked example: 250 → 200
EXAMPLE
250 → 200
The value falls from 250 to 200 — we find the drop as a percentage.
How to calculate a percentage decrease
These steps work for any drop from an old value to a new value.
1
Step 1 · Start
250 → 200
Old value 250, new value 200.
2
Step 2 · Formula
(250 − 200) ÷ 250 · 100
Drop divided by old value, times 100.
3
Step 3 · simplify
50 ÷ 250 · 100
The drop is 50.
4
Step 4 · Result
= 20%
The value fell by 20%.
Why the formula works
A percentage decrease is always relative: it measures the drop against the old value, because that value represents 100%. Dividing the drop (50) by the base (250) gives the share 0.2 — and times 100 makes it 20%. More compactly, you can multiply by the factor (1 − p ÷ 100): "minus 20%" equals multiplying by 0.8.
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