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Difference between revisions of "2008 AMC 12A Problems/Problem 11"

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So, the minimum sum of the <math>5</math> unexposed faces is <math>2\cdot12+1=25</math>.  Since the sum of the numbers on all the cubes is <math>3(32+16+8+4+2+1)=189</math>, the maximum possible sum of <math>13</math> visible numbers is <math>189-25=164 \Rightarrow C</math>.  
 
So, the minimum sum of the <math>5</math> unexposed faces is <math>2\cdot12+1=25</math>.  Since the sum of the numbers on all the cubes is <math>3(32+16+8+4+2+1)=189</math>, the maximum possible sum of <math>13</math> visible numbers is <math>189-25=164 \Rightarrow C</math>.  
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==Solution 2==
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Conversely, maximize the sum. The first two cubes have 4 numbers out facing. Since <math>32>16+8+4+2</math>, 32 must be on the side. There are two distinct (asymmetrical) configurations with 32 on the side, but <math>(32,16,2,1)</math> is the greatest at 51. There are 2 such cubes, so 51*2. The cube on top only hides 1 number, so simply use 1 as the unexposed face. <math>2(51)+32+16+8+4+2=164implies \boxed{\textbf{C)}164}</math>
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~BJHHar
  
 
==See Also==
 
==See Also==
 
{{AMC12 box|year=2008|num-b=10|num-a=12|ab=A}}
 
{{AMC12 box|year=2008|num-b=10|num-a=12|ab=A}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Revision as of 13:22, 12 January 2020

Problem

Three cubes are each formed from the pattern shown. They are then stacked on a table one on top of another so that the $13$ visible numbers have the greatest possible sum. What is that sum?

[asy] unitsize(.8cm); pen p = linewidth(1); draw(shift(-2,0)*unitsquare,p); label("1",(-1.5,0.5)); draw(shift(-1,0)*unitsquare,p); label("2",(-0.5,0.5)); draw(unitsquare,p); label("32",(0.5,0.5)); draw(shift(1,0)*unitsquare,p); label("16",(1.5,0.5)); draw(shift(0,1)*unitsquare,p); label("4",(0.5,1.5)); draw(shift(0,-1)*unitsquare,p); label("8",(0.5,-0.5)); [/asy]

$\mathrm{(A)}\ 154\qquad\mathrm{(B)}\ 159\qquad\mathrm{(C)}\ 164\qquad\mathrm{(D)}\ 167\qquad\mathrm{(E)}\ 189$

Solution

To maximize the sum of the $13$ faces that are showing, we can minimize the sum of the numbers of the $5$ faces that are not showing.

The bottom $2$ cubes each have a pair of opposite faces that are covered up. When the cube is folded, $(1,32)$; $(2,16)$; and $(4,8)$ are opposite pairs. Clearly $4+8=12$ has the smallest sum.

The top cube has 1 number that is not showing. The smallest number on a face is $1$.

So, the minimum sum of the $5$ unexposed faces is $2\cdot12+1=25$. Since the sum of the numbers on all the cubes is $3(32+16+8+4+2+1)=189$, the maximum possible sum of $13$ visible numbers is $189-25=164 \Rightarrow C$.

Solution 2

Conversely, maximize the sum. The first two cubes have 4 numbers out facing. Since $32>16+8+4+2$, 32 must be on the side. There are two distinct (asymmetrical) configurations with 32 on the side, but $(32,16,2,1)$ is the greatest at 51. There are 2 such cubes, so 51*2. The cube on top only hides 1 number, so simply use 1 as the unexposed face. $2(51)+32+16+8+4+2=164implies \boxed{\textbf{C)}164}$

~BJHHar

See Also

2008 AMC 12A (Problems • Answer Key • Resources)
Preceded by
Problem 10 		Followed by
Problem 12
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 12 Problems and Solutions

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