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Financial Mathematics · JSS 1, JSS 2

Percentage increase and decrease

Compare prices, salaries or scores before and after a change.

  • Percentage change = new − oldold × 100%
  • New amount = old × (1 ± r100)

What the letters mean

old
the original amount
new
the amount after the change
r
the percentage rate

Worked example

The price of a loaf rises from ₦2,500 to ₦3,000. Find the percentage increase.

  1. Step 1: Change

    3,000 − 2,500 = ₦500

  2. Step 2: Over the ORIGINAL price

    5002,500 × 100% = 20%

Final answer: 20%

Always divide by the original amount, not the new one.

See it

Play with this picture from the Percentage Increase and Decrease lesson.

Old price = 100%Added: 20%

1 square = 1% of ₦500 = ₦5

Percent20%
Old price₦500
Old price₦500
Increase: 20% of ₦500+ ₦100
New price₦600
Multiplier: 100% + 20% = 120%× 1.2