White & Black Blocks : GMAT Problem Solving (PS)
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# White & Black Blocks

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Manager
Joined: 26 Apr 2010
Posts: 122
Concentration: Strategy, Entrepreneurship
Schools: Fuqua '14 (M)
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Kudos [?]: 129 [0], given: 54

White & Black Blocks [#permalink]

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02 Oct 2011, 13:19
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Question Stats:

67% (01:00) correct 33% (18:19) wrong based on 4 sessions

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Tracy is playing with a set of building blocks. She has three identical black blocks and five identical white blocks. How many different ways are there to stack all eight blocks on top of one another so that no two black blocks are touching?

a) 10
b) 15
c) 20
d) 30
e) 60

OA to follow...thanks!
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Senior Manager
Joined: 11 May 2011
Posts: 372
Location: US
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Re: White & Black Blocks [#permalink]

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02 Oct 2011, 15:59
My answer is - (C)20.

Explanation - 5 white identical White blocks and 3 identical Black blocks neest o be arranged.
for "no two black blocks to touch", 3 Black blocks have 6 places to occupy because 5 of White blocks (alternate positions).

Hence, arrangement = 6P3 = 20.
Let me know OA?

Cheers!
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Re: White & Black Blocks   [#permalink] 02 Oct 2011, 15:59
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# White & Black Blocks

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