Question 744517
This a Pythagorean problem.

The hypotenuse = 15 ft long
One of the sides = 12.5 feet
The base of the ramp represents the remaining side.

Base ^2 + side ^2 = hypotenuse ^2
Base ^2 = hypotenuse ^2 - side ^2
Base ^2 = 15 ^2 - 12.5 ^2
Base ^2 = 68.75
Base    = square root of 68.75
Base =  8.3 ft