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Age 11 to 14 Challenge Level:

Well done to everyone who correctly worked out that the third solution in the hint is correct. From 20 feet above the ground the bottom of the wheel appears horizontal, so it covers the top right corner almost immediately after covering the top left corner. 
Thanks to Julian (Hastingsbury School) who worked out how long the window would be covered for.

If you consider the radius that touches the ground, the angle this makes with the radius 20 feet above the ground is 0.7 degrees, so the wheel turns 1.4 degrees whilst covering the window.

The wheeil travels 100pi miles each time it completes a whole turn, taking pi hours. So the wheel will cover the window for (1.4 / 360) pi hours = 0.0122 hours = 44 seconds. The angle of the tangent to the circle where it first touches the window is less than 1 degree to the horizontal, and so it appears to be coming down horizontally as we expected above!