GATE 2011 CS (CS1) – Question 13

Engineering Mathematics · Probability and Statistics · 1 mark · Multiple choice

Two fair coins are flipped. At least one outcome is known to be a head. What is the probability both are heads?

$1/3$
$1/4$
$1/2$
$2/3$

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Show answer and explanation

Correct answer: (A) $1/3$

Explanation

Conditioning on at least one head leaves equally likely HH,HT,TH. Only HH has two heads, so probability is 1/3, A. This uses the stated event, not observation of one particular designated coin.

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