Question 3149: Comparing Two Directions of Conditional Probability

Original question and givens

The nonempty finite sample space is S={11, 12, 13, 14}, with event family F=𝒫(S). In the displayed order of S, the nonnegative normalized weights are [2/(35), 4/(35), 5/(35), 24/(35)], summing exactly to one. A={11, 12, 13, 14}, B={12, 13}, and both P(A) and P(B) are positive. Are P(A|B) and P(B|A) equal?

• G001 — The nonempty finite sample space is S={11, 12, 13, 14}, with event family F=𝒫(S). In the displayed order of S, the nonnegative normalized weights are [2/(35), 4/(35), 5/(35), 24/(35)], summing exactly to one. A={11, 12, 13, 14}, B={12, 13}, and both P(A) and P(B) are positive. Are P(A|B) and P(B|A) equal?

Definition and scope

Use the original question domain, assumptions and cited MKO definitions. This item has second-context mathematical and bilingual review, with the model, answer and every arithmetic step independently recomputed from the original parameters; the source question remains a candidate.

• mko-conditional-probability

Step-by-step derivation

S={11, 12, 13, 14};A={11, 12, 13, 14};B={12, 13};F=𝒫(S)

events are declared subsets;conditioning needs P(B)>0

A={11, 12, 13, 14};B={12, 13}

A∩B={12, 13}

total weight numerator := sum(2, 4, 5, 24)

total weight numerator = 35

[[11, 2], [12, 4], [13, 5], [14, 24]]

weights sum to 35;normalization verified

B weight numerator := sum(4, 5)

B weight numerator = 9

B probability mass := div(9, 35)

B probability mass = 9/35

A∩B weight numerator := sum(4, 5)

A∩B weight numerator = 9

A∩B probability mass := div(9, 35)

A∩B probability mass = 9/35

P(A|B) := div(9/35, 9/35)

P(A|B) = 1

P(A∩B)=9/35;P(B)=9/35>0

P(A|B)=1

A weight numerator := sum(2, 4, 5, 24)

A weight numerator = 35

A probability mass := div(35, 35)

A probability mass = 1

P(B|A) := div(9/35, 1)

P(B|A) = 9/35

P(A|B)=1;P(B|A)=9/35

equal=No

Reverse checks

• S014 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
• S013 · premise_and_result_audit — Recompute P(B|A) with exact rational arithmetic, not rounded decimals.
• S012 · premise_and_result_audit — Recompute A probability mass with exact rational arithmetic, not rounded decimals.
• S011 · premise_and_result_audit — Recompute A weight numerator with exact rational arithmetic, not rounded decimals.
• S010 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
• S009 · premise_and_result_audit — Recompute P(A|B) with exact rational arithmetic, not rounded decimals.
• S008 · premise_and_result_audit — Recompute A∩B probability mass with exact rational arithmetic, not rounded decimals.
• S007 · premise_and_result_audit — Recompute A∩B weight numerator with exact rational arithmetic, not rounded decimals.
• S006 · premise_and_result_audit — Recompute B probability mass with exact rational arithmetic, not rounded decimals.
• S005 · premise_and_result_audit — Recompute B weight numerator with exact rational arithmetic, not rounded decimals.
• S004 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
• S003 · premise_and_result_audit — Recompute total weight numerator with exact rational arithmetic, not rounded decimals.
• S002 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
• S001 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.

Result

No

The full substitutions and premise checks give the original answer; source identity,format and status are unchanged.

This question review and source binding

Review date: 2026-10-26

Original questions, answers, step expressions, numeric ASTs and historical records are retained. Source candidate labels remain historical and are not overwritten as formal proof.