Aroad has a grade of 28.4%. this means that the road rises 28.4 ft over a horizontal distance of 100 ft. what angle does the hill make with a horizontal line?

If it has a grade of 28.4% then that means the "rise over the run" equals 28.4 / 100.  The trigonometric tangent function is based on the ratio of opposite over adjacent (rise over run).
So, if we look up the arc tangent of .284, we see that it equals an angle of 15.855 degrees.