If a tree is 12 feet tall and has a 16 ft shadow how tall is a tree that casts a 20 ft shadow?

Assuming the shadows are measured at the same time of day and that the trees are on level ground, the tree with a 20-foot shadow is a quarter longer than the tree with a 16-foot shadow. Adding a quarter of the height to 12 feet makes it 15 feet tall.

Alternatively use the tangent ratio which will be opposite (height of 1st tree) over adjacent (its shadow) and multiply it by the adjacent of the 2nd tree:

(12/16)*20 = 15 feet tall.