2001 AMC 10 Problem 5

Question

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

3

4

5

6

7

A line of symmetry folds the shape exactly onto itself

Check each of the twelve shapes for a horizontal, vertical, or diagonal fold line

Checking each pentomino for a reflection line, exactly six have at least one: the straight bar, the plus, the

T

-shape, the

U

-shape, the

V

-shape, and the

W

-shape. The remaining six have no line of symmetry.

Thus, the correct answer is D.

Problem 5 in other years