The square root of 20 is 4.472135... so, you have to solve what of your possible solutions could be the right answer (it's best to use a graphing calculator). We already know it not A, so that's off the list because the square root of 20 is already greater than 4. We then move onto C or 2root5 which we also get 4.472135... (by doing 2 * the square root of 5). We then move onto B and find that 5root2 is 7.07 (by multiplying 5 by the square root of 2). So B is off the plate. Lastly, is 20 = to the square root of 20. No, we already established that in the beginning of the problem. So, overall the answer is C or 2root5.
So 2√5 is the correct answer. It's very important to remember that you can multiply and divide numbers inside of a square root to simplify it or expand it (as I did above) but not add and subtract.
