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'The London Eye' printed from https://nrich.maths.org/
Answer: 500 metres
80 spokes = 4 miles
80 $\times$ radius = 4 miles
20 $\times$ radius = 1 mile
20 $\times$ radius $\approx$ 1600 metres
radius $\approx$ 80 metres
circumference $\approx$ 160$\pi$ metres
$\approx$ 500 metres