You may recognize the dimensions as belonging to the (5, 12, 13) Pythagorean triple.
The ramp is 13 ft long.
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In case you haven't memorized that set of right-triangle dimensions, you can figure them out using the Pythagorean theorem.
(ramp length)² = (bed height)² + (distance from truck)²
(ramp length)² = (5 ft)² +(12 ft)²
(ramp length)² = (25 +144) ft² = 169 ft²
Taking the square root, you have
(ramp length) = √(169 ft²) = 13 ft
