Problem 3 · 2011 Math Kangaroo
Easy
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
If the crossing starts and ends with a white stripe, how many black stripes sit between the white ones?
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Hint 2 of 2
Count the total number of stripes first, then multiply by the 50 cm width.
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Approach: count all stripes, then multiply by width
- The pattern is white, black, white, ... starting and ending white, with 8 white stripes.
- Between/around 8 white stripes there are 7 black stripes, so 8+7=15 stripes total.
- Each stripe is 50 cm = 0.5 m, so the road is 15×0.5 = 7.5 m wide.
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