@mmk2014 I never said approximating was a bad or wrong solution, but for me at least, on many cases it’s harder to verify if the answer is correct. If it works for you, great.
However approximating won’t really work for any non-multiple choice questions (such as SAT grid-ins), or ACT questions where two or more choices are really close together. For me it wasn’t immediately obvious that 31 was out, but C,D,E clearly were.
On some questions though, I’ll agree that it is much faster to approximate. For example:
[Set 4 Question 8](http://www.actstudent.org/sampletest/math/math_04.html)
The area of the square is 10^2 = 100, and the area of each semicircle is a little less than 50 (alternatively, the area of each full circle is less than 100, but greater than 50). Only choice H makes sense.