Answer
$128\ ft$
Work Step by Step
Use the figure and numbers given in the exercise, assume the horizontal gap is $x\ ft$, we have $tan20^\circ=\frac{30}{x}$ and $tan50^\circ=\frac{H-30}{x}$, where $H$ is the height of the right building. Thus we have $\frac{30}{tan20^\circ}=\frac{H-30}{tan50^\circ}$ and $H=30+\frac{30tan50^\circ}{tan20^\circ}\approx128\ ft$
