Probability Questions

Multiple choice
  1. ${\mkern 1mu} \dfrac{5}{9}$
  2. ${\mkern 1mu} \dfrac{4}{9}$
  3. ${\mkern 1mu} \dfrac{1}{2}$
  4. ${\mkern 1mu} \dfrac{1}{3}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let P(odd) = 2p and P(even) = p. Since 3p + 3(2p) = 1, 9p = 1, so p = 1/9. P(even) = 1/3, P(odd) = 2/3. Sum is even if both are even or both are odd. P(even, even) = (1/3)^2 = 1/9. P(odd, odd) = (2/3)^2 = 4/9. Total probability = 1/9 + 4/9 = 5/9.

Multiple choice
  1. $\dfrac{11}{256}$
  2. $\dfrac{15}{256}$
  3. $\dfrac{13}{256}$
  4. $\dfrac{17}{256}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

To get the first HH at flips 7 and 8, the sequence must end in THH. The probability of a specific sequence of length 8 is (1/2)^8 = 1/256. For the sequence to end in THH at position 8, the first 5 flips must not contain HH. The number of such sequences is given by the Fibonacci sequence F(n+2). For n=5, F(7) = 13. Thus, 13/256.

Multiple choice
  1. $241/1456$
  2. $164/4165$
  3. $451/884$
  4. none of these

Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The probability that the first ace appears at position 11 is the number of ways to arrange 48 non-aces in the first 10 spots and an ace in the 11th, divided by total permutations. This is C(48, 10) / C(52, 10) * (4/42) = 164/4165.

Multiple choice
  1. $5/36$
  2. $25/2584$
  3. $36/2584$
  4. $26/37$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Total cards = 20. Cards: 10 E, 5 I, 5 A. We need A, I, E, E, E in that order. P(A) = 5/20. P(I) = 5/19. P(E) = 10/18. P(E) = 9/17. P(E) = 8/16. Multiply: (5/20)(5/19)(10/18)(9/17)(8/16) = 25/2584.

Multiple choice
  1. $\dfrac {52}{169}$
  2. $\dfrac {25}{169}$
  3. $\dfrac {49}{169}$
  4. $\dfrac {24}{169}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Probability of an ace p = 4/52 = 1/13. Probability of no ace q = 12/13. This is a binomial distribution with n=2, p=1/13. P(X=1) = 2 * (1/13) * (12/13) = 24/169. P(X=2) = (1/13)^2 = 1/169. Sum = 25/169.

Multiple choice
  1. $\dfrac{26}{49}$
  2. $\dfrac{32}{49}$
  3. $\dfrac{27}{49}$
  4. $\dfrac{21}{49}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Initial: 5R, 2G. If G is drawn (prob 2/7), urn becomes 6R, 1G. If R is drawn (prob 5/7), urn becomes 4R, 3G. Prob(2nd is R) = (2/7 * 6/7) + (5/7 * 4/7) = 12/49 + 20/49 = 32/49.