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Difference between revisions of "2004 AMC 10A Problems/Problem 16"

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==Solution 1==
 
==Solution 1==
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There are:
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Since there are five types of squares: <math>1 \times 1, 2 \times 2, 3 \times 3, 4 \times 4,</math> and <math>5 \times 5.</math> We must find how many of each square contain the black shaded square in the center.
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*<math>1</math> of the <math>1\times 1</math> squares containing the black square,
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*<math>4</math> of the <math>2\times 2</math> squares containing the black square,
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If we list them, we get that
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*<math>9</math> of the <math>3\times 3</math> squares containing the black square,
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*There is <math>1</math> of all <math>1\times 1</math> squares, containing the black square
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*<math>4</math> of the <math>4\times 4</math> squares containing the black square,
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*There are <math>4</math> of all <math>2\times 2</math> squares, containing the black square
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*<math>1</math> of the <math>5\times 5</math> squares containing the black square.
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*There are <math>9</math> of all <math>3\times 3</math> squares, containing the black square
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*There are <math>4</math> of all <math>4\times 4</math> squares, containing the black square
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*There is <math>1</math> of all <math>5\times 5</math> squares, containing the black square
  
 
Thus, the answer is <math>1+4+9+4+1=19\Rightarrow\boxed{\mathrm{(D)}\ 19}</math>.
 
Thus, the answer is <math>1+4+9+4+1=19\Rightarrow\boxed{\mathrm{(D)}\ 19}</math>.

Revision as of 14:43, 12 February 2017

Problem

The $5\times 5$ grid shown contains a collection of squares with sizes from $1\times 1$ to $5\times 5$. How many of these squares contain the black center square?

2004 AMC 10A problem 16.png

$\mathrm{(A) \ } 12 \qquad \mathrm{(B) \ } 15 \qquad \mathrm{(C) \ } 17 \qquad \mathrm{(D) \ } 19\qquad \mathrm{(E) \ } 20$

Solution 1

Since there are five types of squares: $1 \times 1, 2 \times 2, 3 \times 3, 4 \times 4,$ and $5 \times 5.$ We must find how many of each square contain the black shaded square in the center.

If we list them, we get that

  • There is $1$ of all $1\times 1$ squares, containing the black square
  • There are $4$ of all $2\times 2$ squares, containing the black square
  • There are $9$ of all $3\times 3$ squares, containing the black square
  • There are $4$ of all $4\times 4$ squares, containing the black square
  • There is $1$ of all $5\times 5$ squares, containing the black square

Thus, the answer is $1+4+9+4+1=19\Rightarrow\boxed{\mathrm{(D)}\ 19}$.

Solution 2

We use complementary counting. There are only $2\times2$ and $1\times1$ squares that do not contain the black square. Counting, there are $12$ $2\times2$, and $25-1 = 24$ $1\times1$ squares that do not contain the black square. That gives $12+24=36$ squares that don't contain it. There are a total of $25+16+9+4+1 = 55$ squares possible, therefore there are $55-36 = 19$ squares that contains the black square, which is $\boxed{\mathrm{(D)}\ 19}$.

See also

2004 AMC 10A (Problems • Answer Key • Resources)
Preceded by
Problem 15 		Followed by
Problem 17
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 10 Problems and Solutions

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