I often find myself using decimals and not converting them. In theory it would make sense if the question didn’t specifically say “as a percent” that you couldn’t mark it wrong, however what is the opinion of the board?
If it’s a return question, I always state the following: “Their minimum required return is x%.” I think the grader will have an easier time if they know what your final conclusion is.
I think you’ll be fine - 0.5 is a fair representation of 50% (assuming you don’t put the ‘%’ sign after it) - but try to remember to do it regardless =) I’ve been advised that it is still possible to get full marks even if you don’t get the correct answer as long as you demonstrate you know what you are doing to the marker. IE, fat fingers on the calculator wouldn’t lose you marks if your written method is correct and they can tell from your workings that you knew what was needed. Also, play the exam game. If you get stuck somewhere, just write ‘assuming dollar duration was $1000…’ and carry on.  Don’t quote me on this. PS: Always circle/underline your answer, or write a clear statement as bpdulog does.