Percentages, Ratio and Proportion

Three ways of describing the same idea — comparing quantities — and the discounts, recipes and exchange rates that need them every day.

หลักสูตรแกนกลาง / สสวท.: จำนวนและพีชคณิต (Number and Algebra)ตรงกับ ม.1 บทที่ 7 อัตราส่วน สัดส่วน และร้อยละ
Common Core (US): 6.RP, 7.RP
Cambridge: Lower Secondary Stage 7 — Number: fractions, decimals, percentages, ratio and proportion

5.1 Fractions, decimals and percentages

A percentage is a fraction with a denominator fixed at $100$ . Fixing it is the whole point: comparing $\frac{17}{25}$ with $\frac{13}{20}$ takes work, but $68%$ against $65%$ takes none.

Fraction	Decimal	Percentage
$\frac{1}{2}$	$0.5$	$50%$
$\frac{1}{4}$	$0.25$	$25%$
$\frac{3}{4}$	$0.75$	$75%$
$\frac{1}{5}$	$0.2$	$20%$
$\frac{1}{3}$	$0.333 \dots$	$33.3%$
$\frac{1}{8}$	$0.125$	$12.5%$

Conversions worth knowing without thinking

Moving between the three

1Fraction to decimal: divide the top by the bottom
2Decimal to percentage: multiply by $100$
3Percentage to fraction: put it over $100$ and simplify

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