The sun is in such a position that the man's shadow (4ft) is equal to two-thirds of his actual height (6ft). Using the same scale, it can be taken that the tree's shadow of 18 feet is equal to two-thirds of the tree's actual height, thus :-

18 divided by 2 = 9.

9 x 3 = 27

So the tree is 27 feet high.

18 divided by 2 = 9.

9 x 3 = 27

So the tree is 27 feet high.