I would use ratios.
Let x=height of the tree
FYI 5'6" is = 5.5' (6"/12"=.5)
5.5/11=20/x
Cross multiply
5.5x = 20x11
divide both sides by 5.5
x=20x11/5.5
solve for x and get 40
therefore The Tree is 40' Tall
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