HSPT Question of the Day

1. 0.3
2. 0.5
3. 0.7 (correct answer)
4. 0.9

Explanation: When you encounter probability questions, remember that all possible outcomes must add up to 1 (or 100%). This fundamental rule is key to solving complement problems like this one. If an event has probability $p = 0.3$, then the probability it does not occur is simply $1 - 0.3 = 0.7$. Think of it this way: there are only two possibilities—either the event happens or it doesn't. Since these probabilities must sum to 1, and we know one piece (0.3), we can find the other. Let's examine why the other answers don't work. Choice A (0.3) gives you the probability that the event does occur, not that it doesn't—this is the opposite of what the question asks. Choice B (0.5) would only be correct if the original probability were 0.5, making this a 50-50 situation. Choice D (0.9) would mean the original event had probability 0.1, not 0.3. The correct answer is C (0.7) because $0.3 + 0.7 = 1.0$, satisfying the fundamental probability rule. Study tip: For any probability complement question, use the formula: P(not A) = 1 - P(A). This works because the probability of all possible outcomes must equal 1. When you see "does not occur," "fails," or "complement," immediately think "1 minus the given probability." This pattern appears frequently on standardized tests, so master this simple subtraction and you'll handle these questions quickly and confidently.