Percentages
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Formulas & 30-Second Shortcut Tricks
Formulas
- x% of y = x·y/100 = y% of x
- Percentage change = (New − Old)/Old × 100
- If price rises r%, consumption must fall r/(100+r) × 100 % to keep spend same
- Successive changes a% and b%: a + b + ab/100
Shortcuts
- Fraction table: 1/8 = 12.5%, 1/6 = 16.67%, 1/3 = 33.33%, 3/8 = 37.5%.
- x% of y = y% of x — 8% of 50 = 50% of 8 = 4.
Practice problems
What is 15% of 360?
A price is increased by 30% and then decreased by 10%. What is the net percentage change?
The price of sugar rises by 50%. By how much % must a family cut consumption to keep expenditure unchanged?
What is 35% of 360?
A price is increased by 30% and then decreased by 20%. What is the net percentage change?
The price of sugar rises by 20%. By how much % must a family cut consumption to keep expenditure unchanged?
What is 40% of 800?
A price is increased by 20% and then decreased by 20%. What is the net percentage change?
The price of sugar rises by 20%. By how much % must a family cut consumption to keep expenditure unchanged?
What is 12% of 480?
Percentages Speed Test
1 minute per question. Use the palette to jump around; you'll see time spent on every question at the end.