A 3' * 3' packing case is placed against a wall. A 15' ladder is placed such that it touches the floor, the edge of the packing case and the wall.
The question is:
How high up the wall is the ladder?
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^ |\
| \ | \ | \ 15
? | \
| \ | \ |-------\ | 3 |\ | 3| \
v |_ _ _ | \_ _ _ _