I assume you mean one that is not rational, such as √2. In such a case, you make a reasonable estimate of it's position, and then label the point that you plot. For example, you know that √2 is greater than 1 and less than 2, so put the point at about 1½ (actual value is about 1.4142). For √3, you know the answer is still less than 4, but greater than √2. If both of those points are required to be plotted just make sure you put it in proper relation, otherwise about 1¾ is plenty good (actual value is about 1.7321). If you are going to get into larger numbers, it's not a bad idea to just learn a few roots. Certainly 2, 3, and 5 (2.2361) and 10 (3.1623) shouldn't be too hard. Then for a number like 20, which you can quickly workout is √4•√5 or 2√5, you could easily guess about 4½ (4.4721). They're usually not really interested in your graphing skills on this sort of exercise. They just want you to demonstrate that you have a grasp of the magnitude of irrational numbers.
Catherine could take the original price of $144.97 and multiply it by .09 (9%). This would equal a sales tax of $13.073 which could be rounded to $13.01. Then add the original price of $144.97 to the tax of $13.01 to get a total of $157.98.
