Calculating Probability
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ISEE Middle Level: Mathematics Achievement › Calculating Probability
A single card is drawn from a standard 52-card deck. What is the probability that the card is a heart or a face card (Jack, Queen, or King)?
\(\frac{1}{2}\)
\(\frac{11}{26}\)
\(\frac{3}{13}\)
\(\frac{25}{52}\)
Explanation
There are 13 hearts in a 52-card deck. There are 12 face cards (3 in each of the 4 suits). However, the Jack, Queen, and King of hearts are both hearts and face cards. To avoid double-counting, we use the formula \(P(A \text{ or } B) = P(A) + P(B) - P(A \text{ and } B)\). The number of favorable outcomes is the number of hearts plus the number of face cards minus the number of cards that are both: \(13 + 12 - 3 = 22\). The total number of possible outcomes is 52. Therefore, the probability is \(\frac{22}{52}\), which simplifies to \(\frac{11}{26}\).
A number is randomly chosen from the set of all factors of 72. What is the probability that the chosen number is a multiple of 3?
\(\frac{2}{3}\)
\(\frac{1}{2}\)
\(\frac{3}{4}\)
\(\frac{5}{6}\)
Explanation
First, list all the factors of 72. The factors are 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72. There are a total of 12 factors. Next, identify which of these factors are multiples of 3. The multiples of 3 in this list are 3, 6, 9, 12, 18, 24, 36, 72. There are 8 such numbers. The probability is the ratio of the number of favorable outcomes to the total number of outcomes: \(\frac{8}{12} = \frac{2}{3}\).
A jar contains 5 red marbles, 4 blue marbles, and 3 green marbles. If two marbles are drawn from the jar at random without replacement, what is the probability that both marbles are blue?
\(\frac{2}{11}\)
\(\frac{1}{9}\)
\(\frac{1}{11}\)
\(\frac{1}{3}\)
Explanation
The total number of marbles in the jar is \(5 + 4 + 3 = 12\). The probability of the first marble being blue is \(\frac{4}{12}\). After one blue marble is drawn, there are 11 marbles left, and 3 of them are blue. So, the probability of the second marble being blue is \(\frac{3}{11}\). To find the probability of both events happening, multiply their probabilities: \(\frac{4}{12} \times \frac{3}{11} = \frac{1}{3} \times \frac{3}{11} = \frac{1}{11}\).
The probability that event A occurs is 0.4, and the probability that event B occurs is 0.5. If the probability that both A and B occur is 0.1, what is the probability that neither A nor B occurs?
0.1
0.2
0.8
0.9
Explanation
The probability that either A or B (or both) occurs is given by the formula \(P(A \cup B) = P(A) + P(B) - P(A \cap B)\). Using the given values, \(P(A \cup B) = 0.4 + 0.5 - 0.1 = 0.8\). The event that 'neither A nor B occurs' is the complement of the event that 'either A or B occurs'. Therefore, the probability is \(1 - P(A \cup B) = 1 - 0.8 = 0.2\).
An integer is randomly selected from the integers 1 to 40, inclusive. What is the probability that the selected integer is a multiple of 4 or a multiple of 6?
\(\frac{1}{2}\)
\(\frac{2}{5}\)
\(\frac{1}{4}\)
\(\frac{13}{40}\)
Explanation
There are 40 integers in total. The number of multiples of 4 is \(40 \div 4 = 10\). The number of multiples of 6 is \(40 \div 6 = 6\) with a remainder (the multiples are 6, 12, 18, 24, 30, 36). To find the number of integers that are multiples of 4 or 6, we add the counts and subtract the overlap. The overlap consists of multiples of the least common multiple of 4 and 6, which is 12. The multiples of 12 up to 40 are 12, 24, and 36 (3 multiples). The number of favorable outcomes is \(10 + 6 - 3 = 13\). The probability is \(\frac{13}{40}\).
In a board game, you roll a fair die once. What is the probability of rolling a 6 outcome?
$1/5$ (20%)
$5/6$ (83%)
$1/6$ (17%)
$1/3$ (33%)
Explanation
This question tests middle school mathematics skills, specifically calculating probability from outcomes (aligned with ISEE standards). Probability measures the likelihood of an event occurring, calculated as the ratio of favorable outcomes to total possible outcomes. In this scenario, students must recognize that a fair die has 6 faces numbered 1 through 6, with only one face showing 6 (favorable outcome). The correct answer works by calculating 1/6 or approximately 17%, showing understanding that each face has equal probability. A common distractor like 5/6 might arise from students calculating the probability of NOT rolling a 6 instead. To help students, use visual aids like dice and emphasize that "fair" means each outcome is equally likely.
In gym class, you roll a fair die once. What is the probability of an even outcome?
$1/3$ (33%)
$1/6$ (17%)
$1/2$ (50%)
$2/3$ (67%)
Explanation
This question tests middle school mathematics skills, specifically calculating probability from outcomes (aligned with ISEE standards). Probability measures the likelihood of an event occurring, calculated as the ratio of favorable outcomes to total possible outcomes. In this scenario, students must identify the even outcomes when rolling a fair die: 2, 4, and 6 (3 favorable outcomes) out of 6 total possible outcomes (1, 2, 3, 4, 5, 6). The correct answer works by calculating 3/6 = 1/2 or 50%, showing understanding that half the numbers on a die are even. A common distractor might be 1/3, assuming only two outcomes are even, or misunderstanding what constitutes an even number. To help students, teach them to list all possible outcomes systematically and identify which satisfy the given condition.
You spin a wheel with 8 equal sections: 3 blue, 2 red, 2 green, 1 yellow. What is the probability of blue outcome?
$1/8$ (12.5%)
$5/8$ (62.5%)
$3/5$ (60%)
$3/8$ (37.5%)
Explanation
This question tests middle school mathematics skills, specifically calculating probability from outcomes (aligned with ISEE standards). Probability measures the likelihood of an event occurring, calculated as the ratio of favorable outcomes to total possible outcomes. In this scenario, students must identify that there are 3 blue sections (favorable outcomes) out of 8 total equal sections on the wheel. The correct answer works by calculating 3/8 or 37.5%, accurately representing the proportion of blue sections. A common distractor like 3/5 fails by using only the number of different colors rather than counting all sections. To help students, emphasize the importance of counting all sections when they are equal in size, and practice converting fractions to percentages by multiplying by 100.
You spin a wheel with 10 equal sections: 4 prize, 6 no-prize. What is the probability of prize outcome?
$4/6$ (67%)
$1/10$ (10%)
$6/10$ (60%)
$4/10$ (40%)
Explanation
This question tests middle school mathematics skills, specifically calculating probability from outcomes (aligned with ISEE standards). Probability measures the likelihood of an event occurring, calculated as the ratio of favorable outcomes to total possible outcomes. In this scenario, students must identify that there are 4 prize sections (favorable outcomes) out of 10 total equal sections on the wheel. The correct answer works by calculating 4/10 or 40%, accurately representing the proportion of prize sections. A common distractor like 6/10 might occur if students mistakenly count the no-prize sections instead. To help students, emphasize careful reading to identify what outcome is being asked for, and practice simplifying fractions when appropriate.
You flip a fair coin three times. Outcomes are H or T each time, so 8 total. What is the probability of HHH outcome?
$1/6$
$1/8$
$1/2$
$3/8$
Explanation
This question tests middle school mathematics skills, specifically calculating probability from outcomes (aligned with ISEE standards). Probability measures the likelihood of an event occurring, calculated as the ratio of favorable outcomes to total possible outcomes. In this scenario, students must identify and count the HHH outcome from three coin flips, with 1 out of 8 possible outcomes. The correct answer works by accurately calculating the probability as 1/8, showing a clear understanding of the total outcome space. A common distractor fails by confusing with fewer flips, leading to 1/4. To help students, teach them to list all possible outcomes and practice converting between fractions and percentages. Encourage checking calculations by ensuring the probabilities sum to 1 or 100%.