I'm not sure whether this got resolved, but I checked some things out.
The default format for a student's answer is a Formula MathObject, not a Fraction MathObject. Hence, WeBWorK gets 0.2, not 1/5, in the standard answer checker. So the ultimate issue is the parser that interprets the student's answer, and the answer is as simple (and as complicated) as finding the proper parameter and what its value should be.
In 2014, Paul Pearson mentioned something along similar lines:
The default format for a student's answer is a Formula MathObject, not a Fraction MathObject. Hence, WeBWorK gets 0.2, not 1/5, in the standard answer checker. So the ultimate issue is the parser that interprets the student's answer, and the answer is as simple (and as complicated) as finding the proper parameter and what its value should be.
In 2014, Paul Pearson mentioned something along similar lines:
During our workshop session this morning, one question in the chat window was about how to
(1) display a reduced fraction in the text of a webwork question and
(2) require that students enter their answer as a reduced fraction.
This would probably give you a solution that is quicker to implement.