Four road signs are shown. Exactly one of them has two lines of symmetry. Which one is it?
Text description of the figure
Four outlined shapes side by side, labelled A to D: a plain rectangle, an isosceles triangle, a five-pointed star, and an arrow pointing right.
Hints
Take them one at a time. The first gives nothing away.
1A nudge
Try folding each shape every way you can.
2The strategy
A line of symmetry has to make the two halves land exactly on each other. Test the vertical fold, the horizontal fold, and both diagonals on every shape.
3The full solution
The rectangle folds along its vertical midline and along its horizontal midline, so it has two. Its diagonals do not work unless it is a square. The triangle has one, the star has five, the arrow has one.
Solution— no shortcut on this one
Test the candidate folds one at a time on each shape: vertical, horizontal, and the two diagonals. The rectangle matches on the vertical and the horizontal midlines but not on its diagonals, so it has exactly two. The isosceles triangle has one, the five-pointed star has five, and the arrow has one. The transferable idea: check every candidate fold rather than judging by how symmetric a shape looks.
Why this is on the test: Symmetry questions are graded by whether a student tests the folds or eyeballs the picture, and eyeballing fails on the rectangle every time.