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Middle school⚡Mental math●○○○○· 2 min

%

Percent ↔ decimal: shift the decimal point 2 places

20% = 0.20. 3.5% = 0.035. The 2-second rule that unlocks everything else (proportionality, discounts, tax).

The rule

A percentage is a division by 100. In other words, you shift the decimal point 2 places to the left.

Mental examples

$25\%$ → 0.25
$7\%$ → 0.07
$150\%$ → 1.5
$0.5\%$ → 0.005

To apply a percentage

$20\%$ of $80 = 0.20 \times 80 = 16$ . Faster than “divide by 100 and multiply by 20”.

The 5 percentages to know by heart

%	Decimal	Fraction
$10\%$	0.1	1/10
$20\%$	0.2	1/5
$25\%$	0.25	1/4
$50\%$	0.5	1/2
$75\%$	0.75	3/4

Spotting the fraction makes it even faster: $25\%$ of $80 = 80/4 = 20$ .

?Your turn

A jacket costs $60. It's on sale −30%. What does it cost now?

#percentages#decimal#conversion

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