For the ACT and SAT tests, sometimes you don't even have to know how to find the answer, but rather, all you need is to know how to eliminate wrong answers. There are occasions where even if you know how to solve the problem, it actually takes more time to figure out the right answer by solving the problem than it does to find the correct choice by eliminating incorrect answers.
ACT: Never leave any answers blank, since you are not penalized for wrong answers.
SAT: For multiple choice questions, always guess if you can eliminate 2 or more incorrect answer choices. Never guess if you can't eliminate any incorrect answer choices. The reason is that for incorrect answer choices 1/4 point is subtracted from your score. Since there are 5 answer choices, you have a 1/5 (%20) chance of getting the right answer versus the prospect of getting a (%25) point reduction. However, if you eliminate 2 incorrect choices, then you have a 1/3 (%33) chance of getting the right answer, versus the prospect of losing 1/4 (%25) of a point. For an explanation of this phenomenon, look up the probability and statistics concept of "Expected Value".
AP Calculus: Coming soon