Since this is a mathematics question, let's assume the intention was to find:

sqrt(aduck) which would make perfect sense if aduck was just a base 31 number. So let's convert to base 10 and compute:

sqrt(aduck) = sqrt(10*31^4 + 13*31^3 + 30*31^2 + 12*31 + 20) = 3106.72094 (decimal)

It could also be a higher base, but with no additional information this is the minimum.

Converting back to base 31 the answer is approximately 377.