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White & Black Blocks

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Manager
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Joined: 26 Apr 2010
Posts: 126
Concentration: Strategy, Entrepreneurship
Schools: Fuqua '14 (M)
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White & Black Blocks [#permalink] New post 02 Oct 2011, 13:19
00:00
A
B
C
D
E

Difficulty:

(N/A)

Question Stats:

67% (01:00) correct 33% (18:19) wrong based on 3 sessions
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
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Re: White & Black Blocks [#permalink] New post 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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