Problem 3 · 2025 Math Kangaroo
Easy
Spatial & Visual Reasoning
net-foldingcareful-counting
The left and right parts of a three-part brochure each contain four transparent windows. If these two parts are folded onto the middle part, some of the numbers written on the middle part are visible through the windows. What is the sum of the visible numbers when the brochure is folded?
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Answer: D — 14
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Hint 1 of 3
Both side panels fold over the middle, stacking on top of each other, so each panel reflects left-right as it closes.
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Hint 2 of 3
A middle number shows only if BOTH the left panel and the right panel have a window over that same cell.
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Hint 3 of 3
Find the windows each panel lands on after folding, then keep only the cells where the two sets of windows overlap.
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Approach: intersect the two panels' window positions
- Both flaps fold inward and cover the whole 3×3 middle, so a number is visible only where the left flap AND the right flap both have a window (you must see through both layers).
- Folding the left flap (it reflects) puts its windows over the middle's two right columns; folding the right flap puts its windows over the middle's two left columns — they overlap only in the centre column.
- There the visible numbers are 9 (top) and 5 (middle), so the sum is \(9+5=14\), answer D.
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