There are two basic methods for solving a multiple choice problem:
(1) solve the problem, then select the correct answer
(2) eliminate the wrong answers (called 'distractors') to select the correct answer (called the 'key')
Often, especially on standardized tests, method  is much faster and much better. So, you should learn it.
One of the skills that helps method  is estimating. It uses the information in the problem along with some extra information that you know to quickly find a number that is close to the answer. In fact, it may be close enough to immediately select the correct answer to a multiple-choice question.
PLZ let me illustrate: In making curtain from cloth, the cloth is to be folded and sewn so that it has a fairly snug fit around a 5/8-inch curtain rod. Where should it be sewn? Well, we want to fold the cloth and sew so that the circumference is a little more than the circumference of the curtain rod. But, how much is that? Well, the value of Pi (3/14 or 22/7 estimates it) is a little more than 3; 3 times 5/8 times 3 is 15/8, which is almost 16/8 = 2. So, folding the cloth and sewing 1" from the fold (2" of cloth) produces a diameter slightly bigger than the curtain rod.
Estimating is often done "in your head" or with few calculations.
For this problem, we know that (√3)*(√8) = (√24), which is just a little less than (√25)=5. If one of the distractors is close to 4.9, select it and quickly move on to the next problem.