Solve the Problem $20 \%$ Grade, or Slope $18 \mathrm { ft }$

Multiple Choice

Solve the problem.
-A motorcycle daredevil is planning a stunt to perform at a county fair. A ramp must be built to give him a $20 \%$ grade, or slope. If the vertical height at the end of the ramp must be $18 \mathrm { ft }$ to assure that the stunt is a success, what must be the length of the horizontal run?

A) $90 \mathrm { ft }$
B) $3.24 \mathrm { ft }$
C) $15 \mathrm { ft }$
D) $324 \mathrm { ft }$