The Down and Out Sponge

A semi-circular sponge has a diameter of 20cm.

Quite a nice way to begin sponging a window is to start in one corner, as shown below.

Now slide the top of the sponge down towards the corner of the window as you simultaneously slide the bottom of the sponge out and away from the corner…

 

This way, the diameter always stays in contact with the two edges of the window and you don’t miss any bits.

The Problem: 

What area of the window is wiped clean as you make this first, elegant move?

 

Open the File as a Word Document

 

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Last modified: June 18, 2007