2007 iTest Problems/Problem 4

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Problem

Star flips a quarter four times. Find the probability that the quarter lands heads exactly twice.

$\mathrm{(A)}\,\frac{1}{8}\quad\mathrm{(B)}\,\frac{3}{16}\quad\mathrm{(C)}\,\frac{3}{8}\quad\mathrm{(D)}\,\frac{1}{2}$

Solution

The number of arrangements of the group of letters HHTT is $\frac{4!}{2!*2!}=6$ , so the probability that two of the flips are heads is $\frac{6}{2^4}=\frac{3}{8} \Rightarrow C$

2007 iTest (Problems)
Preceded by: Problem 3	Followed by: Problem 5
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2007 iTest Problems/Problem 4

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