The geometry of the problem can be modeled by a right triangle with hypotenuse 16 ft and one side length of 4 ft. If x represents the height of the ladder on the building, then the Pythagorean theorem tells us ...
x^2 + (4 ft)^2 = (16 ft)^2
x^2 = 240 ft^2 . . . . . . subtract 16 ft^2
x ≈ 15.5 ft . . . . . . . . . . take the square root
The top of the ladder is about 15.5 ft above the ground.
