2001 AMC 10 Problems/Problem 5

Problem

How many of the twelve pentominoes pictured below at least one line of symmetry?

$\textbf{(A)}\ 3 \qquad \textbf{(B)}\ 4 \qquad \textbf{(C)}\ 5 \qquad \textbf{(D)}\ 6 \qquad \textbf{(E)}\ 7$

Solution

Here is the picture: http://www.artofproblemsolving.com/Forum/download/file.php?id=6659&&mode=view

The ones with lines over the shapes have at least one line of symmetry. Counting the number of shapes that have line(s) on them, we find $\boxed{\textbf{(D)} 6}$ pentominoes.

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2001 AMC 10 Problems/Problem 5

Problem

Solution

Solution