Final answer:
The assertion that percents are ratios and not measurements and that they should be written without a hyphen before a noun is false. Percentages can function as a measurement of proportion and the use of hyphens is context-dependent in English grammar.
Step-by-step explanation:
The statement 'Number + noun before a noun but not after. Percents are ratios, not measurements. Always write these expressions without the hyphen.' is False. When writing about percentages in English, the convention differs. Typically, when a number and a percentage are used before a noun, they are often hyphenated, like in 'a 10-percent increase.' When they appear after a noun, they are typically not hyphenated, as in 'the increase is 10 percent.' Furthermore, percents can be seen as specific types of ratios that describe how many parts per hundred there are of something, so saying they are not measurements could be misleading; they measure a proportion relative to a hundred.
Calculating percents involves expressing a fraction as a ratio with a denominator of 100, as in '25%' means 25/100 or 25 per 100. Converting percents to fractions involves writing the number as over 100 and simplifying if needed. Whereas percentage, ratio, and fraction are related concepts used to explain relationships between numbers, an index number is used to simplify data comparisons and does not carry units like percentages.
It's also important to remember that trailing zeros in numbers with decimal points are significant in conveying precision. This rule applies regardless of whether the zeros occur before or after the decimal point, as it can affect the measurement's accuracy and should be noted when dealing with percentages.