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'Walk the Plank' printed from https://nrich.maths.org/
The figure on the right shows the top left-hand corner of the
complete diagram. Note the symmetry which leads to the three
measurements of $\frac{1}{2}$. Thus the diagonal of the square can
be divided into three portions of lengths:
$\frac{1}{2}$, $x$ and $ \frac{1}{2}$ respectively.
The length of the diagonal $= \sqrt{10^2 + 10^2} = \sqrt{200} = 10
\sqrt{2}$.
So $x$ = $10\sqrt{2} - 1$.