Bluebook SAT Test Probability and Conditional Probability — Questions and Answers
Question 1: A bag contains 5 red marbles, 3 blue marbles, and 2 green marbles. If one marble is drawn at random, what is the probability that it is NOT green?
- 1/5
- 4/5 (Correct answer)
- 2/5
- 3/5
Correct answer: 4/5
There are 8 non-green marbles out of 10 total, so P(not green) = 8/10 = 4/5.
Question 2: A survey of 200 students found that 120 play sports and 80 play an instrument. If 40 students do both, how many students do neither?
- 40 (Correct answer)
- 80
- 120
- 160
Correct answer: 40
Using inclusion-exclusion: sports or instrument = 120 + 80 − 40 = 160, so neither = 200 − 160 = 40.
Question 3: The two-way table shows pet ownership among 150 households.
Cat No Cat
Dog 30 45
No Dog 25 50
Given that a household has a dog, what is the probability it also has a cat?
- 2/5 (Correct answer)
- 2/3
- 1/5
- 3/10
Correct answer: 2/5
There are 30 + 45 = 75 dog households; 30 have both, so P(cat | dog) = 30/75 = 2/5.
Question 4: A standard die is rolled twice. What is the probability of getting a 4 on the first roll and a number greater than 2 on the second roll?
- 1/9 (Correct answer)
- 1/6
- 2/9
- 1/3
Correct answer: 1/9
P(4) = 1/6 and P(>2) = 4/6 = 2/3; since independent, multiply: (1/6)(2/3) = 2/18 = 1/9.
Question 5: In a class of 30 students, 18 passed the math test and 12 passed the science test. If 6 students passed both, what is the probability that a randomly selected student passed at least one test?
- 4/5 (Correct answer)
- 3/5
- 2/3
- 1/2
Correct answer: 4/5
Students passing at least one = 18 + 12 − 6 = 24; P = 24/30 = 4/5.
Question 6: A jar has 4 red and 6 blue chips. Two chips are drawn without replacement. What is the probability that both chips are red?
- 2/15 (Correct answer)
- 4/25
- 8/25
- 1/5
Correct answer: 2/15
P(1st red) = 4/10; P(2nd red | 1st red) = 3/9; product = 12/90 = 2/15.
Question 7: The table below shows 300 survey responses by age group and preference.
Prefer A Prefer B
Under 30 90 60
30 and over 80 70
What is the probability that a randomly chosen respondent under 30 prefers A?
- 3/5 (Correct answer)
- 9/25
- 9/20
- 3/10
Correct answer: 3/5
There are 90 + 60 = 150 respondents under 30; 90 prefer A, so P = 90/150 = 3/5.
Question 8: A two-way table records 400 employees by department and satisfaction level.
Satisfied Neutral Dissatisfied
Engineering 80 40 30
Marketing 60 60 50
Operations 40 20 20
What is the probability a randomly selected employee is from Marketing and Dissatisfied?
- 1/8 (Correct answer)
- 5/40
- 1/10
- 1/4
Correct answer: 1/8
Marketing & Dissatisfied = 50 out of 400 total; P = 50/400 = 1/8.
Question 9: P(A) = 0.6 and P(A and B) = 0.18. If A and B are independent, what is P(B)?
- 0.30 (Correct answer)
- 0.18
- 0.42
- 0.78
Correct answer: 0.30
Since A and B are independent, P(B) = P(A and B)/P(A) = 0.18/0.6 = 0.30.
Question 10: A box has 3 red, 5 white, and 2 black balls. One ball is drawn and not replaced; then a second is drawn. What is the probability the second ball is red, given the first was white?
- 1/3 (Correct answer)
- 3/10
- 3/9
- 1/4
Correct answer: 1/3
After removing a white ball, 9 balls remain: 3 red; P = 3/9 = 1/3.
Question 11: A game wheel has 4 red, 3 blue, and 1 green section. A player spins twice. What is the probability of getting green both times?
- 1/64 (Correct answer)
- 1/8
- 1/16
- 3/64
Correct answer: 1/64
P(green) = 1/8; for two independent spins, P = (1/8)² = 1/64.
Question 12: In a class of 25 students, the probability that a randomly chosen student plays guitar is 0.32. What is the expected number of guitar players?
Correct answer: 8
Expected value = 25 × 0.32 = 8.
Question 13: The two-way table shows 500 people by exercise frequency and BMI category.
Normal BMI High BMI
Exercise daily 180 70
Rarely exercise 90 160
What fraction of people with a Normal BMI exercise daily?
- 2/3 (Correct answer)
- 9/10
- 3/5
- 18/27
Correct answer: 2/3
Normal BMI total = 180 + 90 = 270; exercise daily = 180; P = 180/270 = 2/3.
Question 14: A bag has 7 green and 3 yellow tokens. Two tokens are drawn with replacement. What is the probability of drawing one of each color?
- 42/100 (Correct answer)
- 21/50
- 3/10
- 49/100
Correct answer: 42/100
P(green then yellow) + P(yellow then green) = (7/10)(3/10) + (3/10)(7/10) = 42/100 = 21/50.
Question 15: A class has 24 students. The probability of any student being absent on a given day is 0.05. What is the expected number of absent students?
- 1.2 (Correct answer)
- 1.0
- 0.5
- 2.4
Correct answer: 1.2
E = n × p = 24 × 0.05 = 1.2.
Question 16: The two-way table shows 200 voters by party and stance on a bill.
Favor Oppose
Democrat 70 30
Republican 20 80
Given that a voter favors the bill, what is the probability the voter is a Republican?
- 2/9 (Correct answer)
- 1/5
- 4/9
- 2/11
Correct answer: 2/9
Total favoring = 70 + 20 = 90; Republicans favoring = 20; P = 20/90 = 2/9.
Question 17: Cards numbered 1 through 20 are shuffled. One card is drawn at random. What is the probability the card shows a multiple of 3 or a multiple of 5?
- 2/5 (Correct answer)
- 1/2
- 9/20
- 7/20
Correct answer: 2/5
Multiples of 3: {3,6,9,12,15,18} = 6; multiples of 5: {5,10,15,20} = 4; overlap {15} = 1; union = 9; P = 9/20.
Question 18: A bag has 5 red and 5 blue chips. Two chips are drawn without replacement. What is the probability both are the same color?
- 4/9 (Correct answer)
- 1/2
- 2/5
- 5/9
Correct answer: 4/9
P(RR) = (5/10)(4/9) = 20/90; P(BB) = (5/10)(4/9) = 20/90; total = 40/90 = 4/9.
Question 19: A company's hiring data shows: 40% of applicants pass the written test and, of those, 30% pass the interview. What percent of all applicants get hired?
- 12% (Correct answer)
- 30%
- 40%
- 70%
Correct answer: 12%
P(pass written and interview) = 0.40 × 0.30 = 0.12 = 12%.
Question 20: The two-way table shows 300 students by study method and test result.
Pass Fail
Study group 110 40
Study alone 90 60
What is the probability a student who failed studied alone?
- 3/5 (Correct answer)
- 1/3
- 2/5
- 3/10
Correct answer: 3/5
Total failing = 40 + 60 = 100; studied alone and failed = 60; P = 60/100 = 3/5.
Question 21: A number is selected at random from 1 to 50. What is the probability it is divisible by both 4 and 6?
- 2/25 (Correct answer)
- 1/10
- 1/12
- 3/50
Correct answer: 2/25
LCM(4,6) = 12; multiples of 12 up to 50: {12, 24, 36, 48} = 4; P = 4/50 = 2/25.