Good question! It really depends. Generally speaking, answers should be simplified to their simplest form.
When you are comparing your solution with the potential answer choices on the test, it is super important that you know how to simplify to compare your answers. In the case of the sample question in this lesson, it is likely that 2√5 is going to be one of the options, not √20.
If you would like some additional practice with simplifying roots, you should check out the following lesson and blog posts that we have to offer (Yes, these are GMAT blog posts, but they are relevant and helpful)!