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 http://www.finn-aj.freeserve.co.uk)

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.

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!)