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 for downloadable math lessons you might want to have a look at  Math Made Simple