Problem 9 · 2013 Math Kangaroo
Medium
Geometry & Measurement
careful-countingspatial-reasoning
How many triangles can be seen in the picture on the right? (Be careful! A triangle can also be made by joining several smaller triangles together.)
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Answer: B — 10
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Hint 1 of 3
Count the smallest single triangles first, and mark each one as you count it.
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Hint 2 of 3
Then hunt for the bigger triangles that are made by joining two or more small ones together.
Still stuck? Show hint 3 →
Hint 3 of 3
Add the small count and the big count, and be careful not to count the same triangle twice.
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Approach: count the smallest triangles, then add the larger ones built from them
- First count every smallest single triangle in the picture.
- Then look for the larger triangles formed by joining several of the small ones, and count those too.
- Adding the small and the large together gives 10 triangles, which is answer B.
Mark:
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