Problem:
Each half of this figure is composed of red triangles, blue triangles and white triangles. When the upper half is folded down over the centerline, pairs of red triangles coincide, as do pairs of blue triangles. There are red-white pairs. How many white pairs coincide?
Answer Choices:
A.
B.
C.
D.
E.
Solution:
All six red triangles are accounted for, so the two unmatched upper blue triangles must coincide with lower white triangles. Since one lower white triangle is matched with a red triangle and two are matched with blue triangles, there are five left and these must match with upper white triangles.
It may be helpful to construct a diagram such as this:
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The problems on this page are the property of the MAA's American Mathematics Competitions