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Probability & Counting (Advanced)

Harder items at competition-adjacent depth.

Phase 2 · JEE

Probability & Counting: core idea

Bayes intro.

Example problem: hint and answer

Two dice: P(sum=7)P(\text{sum}=7)?

Hint: Six outcomes. Answer: 1/6.

Practice (9 problems — type your answers)

  • 1. Hypergeometric: 5 red in 10, draw 2: P(both red)P(\text{both red})?

  • 2. Bayes: prior 1/21/2, likelihoods 0.80.8 and 0.40.4: posterior for first hypothesis given positive? Enter as reduced fraction a/ba/b of form?

  • 3. Derangements !3!3?

  • 4. Geometric p=1/2p=1/2: P(X=3)P(X=3) first success on 3rd?

  • 5. Var of die roll 1..61..6?

  • 6. Poisson λ=2\lambda=2: P(X=0)P(X=0)?

  • 7. Cov of independent X,Y?

  • 8. Negative binomial: trials until 2nd success p=1/2p=1/2: P(X=3)P(X=3)?

  • 9. Markov: from A stay 0.7, to B 0.3; start A: P(A)P(A) after 1 step?

Probability & Counting Practice Problems — Advanced