A ladder that is 21 feet long is propped against a building. The bottom of the ladder was placed 4 feet from the base of the building. How high up on the building does the ladder reach? Round the answer to the nearest tenth of a foot.

The ladder, wall, and ground form a right triangle, with the ladder as the 21-foot hypotenuse and the ground distance as one 4-foot leg. By the Pythagorean theorem, the height is √(21² − 4²) = √425 ≈ 20.6 feet. The ladder reaches 20.6 feet up the building, choice C.

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