Trig problem #9

This is the first of our Guest Trig Problems.  It was sent in by Tony Finn from East Riding of Yorkshire, England.  (Visit his web site

As always, I'll need an explanation as to how you arrived at your answer. (. . .too many CAD programs out there.)

Send your answer to me, vent your frustrations directly to Tony.

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Tony writes "Have you seen this one?
A twenty foot ladder leans up against a perpendicular wall and just
touches the outer top edge of a 6'x6'x6' cube packing case pushed up
close to the wall. How far up the wall is the top of the ladder?
(It looks simple, but it ain't!)