DRIVING DOWN A 10% GRADE, THEN YOU DESCEND TO 20 FEET BELOW YOUR STARTING POINT. wHAT CONCEPT DOES THE GRADE RELATE TO?

grade is the slope of the road. That is, the change in height divided by the change in horizontal distance.

In this case, descending 20 feet means you have moved horizontally 200 feet.

Doesn't sound like much, but even the steepest roads seldom exceed a 6% grade.