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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

See Also

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