Step-by-step explanation:
maybe you missed something in the problem definition or in the answer options, but the answer options do not contain any right answer to the described problem.
20 - sqrt(80) = 20 - sqrt(16×5) = 20 - 4×sqrt(5) =
= 11.05572809...
it is simple to estimate, in what dimension the result should be : 80 is very close to 81. so the sqrt(80) is also very close to the sqrt(81), which is 9.
20 - 9 = 11.
so, all the negative answer options are wrong by default.
and the only positive one is 2×sqrt(5) = 4.472135955...
additional thoughts :
20 = 4×5 or 4×sqrt(5)×sqrt(5)
so, we could write the problem as
4×sqrt(5)×sqrt(5) - 4×sqrt(5)
which could be simplified to either
4×sqrt(5)×(sqrt(5) - 1)
or
4×(5 - sqrt(5))
