According to the information given, the ramp would be 50 feet high. (However, that seems really high. Did you type your problem correctly?)
If you draw a picture, you will see that you can set up a cosine ratio to solve this problem. With a 30 degree angle of depression, the angle we are going to use 60 degrees.
We can set up the following equation:
cos(60) = x / 100
0.5 = x / 100
50 = x
