A scaffold builder needs to access a work platform located 15 feet (4.57 meters) above the ground. Using a portable extension ladder, what is the minimum required ladder length to safely reach this platform, assuming proper angle and extension?

Correct answer: D. 20 feet (6.10 meters)

To determine the ladder length, you need to consider the 4:1 ratio (for every 4 feet of vertical height, the base should be 1 foot away horizontally) and the 3-foot extension above the landing. The working height is 15 feet. To achieve a 3-foot extension, the ladder must reach 15 + 3 = 18 feet. Using the 4:1 rule, the ladder's vertical reach is approximately 4/√(1²+4²) = 4/√17 = 4/4.12 ≈ 0.97 times its length. So, a 20-foot ladder would provide a vertical reach of 20 * 0.97 = 19.4 feet, allowing for the 18-foot vertical requirement and a safe angle. A simpler approximation is: (15 ft + 3 ft extension) * 1.25 (for the 4:1 ratio) = 18 ft * 1.25 = 22.5 ft, but since extension ladders overlap, a 20-foot ladder is generally considered the minimum for this height given standard ladder lengths and the 3-foot extension rule. More precisely, for a 15-foot landing height, you need the ladder to extend 3 feet above, so the working length against the wall is 18 feet. With a 4:1 ratio (which means the ladder length is about 1.03 times the vertical height for practical purposes, or simply using the Pythagorean theorem, L = √(H^2 + (H/4)^2)), a ladder needs to be at least 18 feet vertically. A 20-foot extension ladder, when properly overlapped, typically has a maximum working length of 17-18 feet, making it the minimum suitable choice for this scenario.