NOTE; this is NOT my answer. I dont know how to do this problem, but i thought this would help you.
Man's height IS TO Man's shadow AS Tree's height IS TO Tree's shadow
Let the tree's height be x. Then we have
6 IS TO 4 AS x IS TO 18
Replace the "IS TO"'s by " / " and the "AS" by " = "
6/4 = x/18
Cross multiply:
4x = 6(18)
4x = 108
x = 27 feet
basically the tree is 27 feet
