Problem 1 · 2011 Math Kangaroo
Medium
Arithmetic & Operations
off-by-one
A zebra crossing has alternating white and black stripes, each 50 cm wide. The first stripe is white and the last one is white. The zebra crossing in front of our school has 8 white stripes. How wide is the road?
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Answer: B — 7.5 m
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Hint 1 of 2
With the first and last stripe both white, the black stripes sit in the gaps between whites.
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Hint 2 of 2
Count how many stripes there are in total, then multiply by 50 cm.
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Approach: count stripes from the white count
- With 8 white stripes and a black stripe in each gap between them, there are 7 black stripes.
- Total stripes = 8 + 7 = 15, each 50 cm wide.
- Road width = 15 × 50 cm = 750 cm = 7.5 m.
- So the road is 7.5 m wide, choice (B).
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