Walk the plank

A rectangular plank fits neatly inside a square frame when placed diagonally. What is the length of the plank?
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Problem



Image
Walk the Plank


The diagram shows a 1 by x rectangular plank

which fits neatly inside a 10 by 10 square frame.

What is the value of x?
 


If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.