Fractions, decimals, and percentages

The same value, three ways to write it:

Fraction	Decimal	Percentage
1/2	0.5	50 %
1/4	0.25	25 %
3/4	0.75	75 %
1/5	0.2	20 %
1/3	0.333…	33.3 %

Fraction → decimal: divide the numerator by the denominator (3/4 = 3 ÷ 4 = 0.75). Decimal → percentage: multiply by 100 (0.75 → 75 %).

“Percentage” means “per hundred”. 75 % = 75 out of 100.

What proportion is it? An AI model answered 43 out of 50 customer cases correctly. What percentage is that?

43/50 = 43 ÷ 50 = 0.86 = 86 %.

Shortcut: convert the denominator to 100. 50 · 2 = 100, so 43 · 2 = 86 → 86/100 = 86 %.

You do this calculation constantly in AI: accuracy, proportion correct, share of data of a certain type. A number without a denominator means nothing — “40 correct” is excellent out of 40 and poor out of 400.