head or tail ?

Probability Level 2

Box $A$ contains $4$ blue and $3$ white marbles while Box $B$ contains $2$ blue and $9$ white marbles. A fair coin is tossed. If the coin turns up heads, a marble is chosen from Box $A$ ; if it turns up tails, a marble is chosen from Box $B$ . Find the probability that a blue marble is chosen.

\dfrac{58}{77}

\dfrac{2}{77}

\dfrac{29}{77}

None of these.

1 solution

A Former Brilliant Member
Oct 1, 2017

$P=\dfrac{1}{2}\left(\dfrac{4}{4+3}\right)+\dfrac{1}{2}\left(\dfrac{2}{2+9}\right)=\dfrac{2}{7}+\dfrac{1}{11}=\dfrac{29}{77}$

head or tail ?

1 solution

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