I don't think you really want to be using a List object for this, as the List checker will do syntax checking and error reporting appropriate for lists, while the student is supposed to enter a single real. If they enter a list, your checker will fail. Also, the correct answer will be shown as a list rather than a single number.
I recommend using a Real with a custom checker that uses an Interval (see my response below).
WeBWorK Main Forum
Checking if an answer lies in an interval
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