#### Answer

$90\ ft$

#### Work Step by Step

Step 1. Using the given figure, set the middle of bridge as the origin. We can write the general form of the equation for the parabola as $x^2=4py$
Step 2. As the point $(400,160)$ is on the curve, we have $400^2=4p(160)$, which gives $p=250$
Step 3. At 100 feet from the right tower, we have $x=400-100=300$. Thus $y=\frac{x^2}{4p}=\frac{300^2}{4(250)}=90\ ft$