Problem 5 · 2018 Math Kangaroo
Easy
Spatial & Visual Reasoning
shadows-projections
Theodor has built this tower out of discs. He looks at the tower from above. How many discs does he see?
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Answer: C — 3
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Hint 1 of 3
Imagine your eye is a bird looking straight down on the top of the tower.
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Hint 2 of 3
A disc hides under any wider disc that sits above it — you only see the edges that poke out.
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Hint 3 of 3
Look from the top disc downward and count each rim that sticks out past the ones above it.
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Approach: look straight down and count the rims that poke out
- Looking straight down, a disc is hidden if a wider disc sits on top of it.
- Starting from the top, you can see the top disc, plus each lower disc whose edge sticks out beyond everything above it.
- Exactly three rims poke out, so from above Theodor sees 3 discs.
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