@mweston
Numeric response questions only accept decimal representations of the number (or an approximation of the number). They do not accept fractions or other symbolic representations like √5. There is some support for scientific notation. The correctness of the response is compared to the decimal form of the answer. There is also internationalization support, so if you're in a country that uses , for the decimal point and . for the separator, it would still work.
The fill-in-the-blank question type has been the long-recommended standard for math content, but it is a poor workaround.
In Legacy Quizzes, you have to allow for all of the multiple ways you want to accept -- exactly. 0.5 and .5 and 0.50 are different. You could have 1/2 but you could also have 2/4 or 3/6. sin(30°) or cos π/3 are also correct, but less likely to show up 
In New Quizzes, there is a regular expression, so you could mark the leading 0 as optional, /^(0?\.50*|1\/2)$/
You can also allow for similar answers, but that is probably not appropriate in this case.
There is a feature idea open for allowing fractions in a response, although it's about formula questions: https://community.canvaslms.com/ideas/17209-new-quizzes-responses-in-fractions-for-formula-questions
