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Explanation:
The diagram is shown below. We know the vertical part of the triangle is 13 inches, which is the side opposite the reference angle 13 degrees. The adjacent side is unknown. We'll call it x. This is how long the base of the ramp is, which is the horizontal distance along the entire ramp. This distance is on the ground. The ramp itself is the hypotenuse but it seems like your teacher isn't wanting to know this value. So we'll ignore the hypotenuse.
We'll use the tangent rule to connect the opposite and adjacent sides.
tan(angle) = opposite/adjacent
tan(13) = 13/x
x*tan(13) = 13
x = 13/tan(13)
x = 56.309186365694
x = 56 inches is the approximate horizontal distance underneath the ramp
