Okay, here we have this:
Heron's Formula: Area=√(s(s-a)(s-b)(s-c)) and s=(a+b+c)/2
Replacing we obtain:
s=(10+16+20)/2=46/2=23
Using this in the area equation:
Area=√(s(s-a)(s-b)(s-c))=√(23(23-10)(23-16)(23-20))=√(23(13)(7)(3))=√6279=79.24
Finally we obtain that the correct answer is 79.24.