6 copies of each title and reading level, 648 books total. Ladders Science consists of high interest science topics for Life, Earth, Physical Science, and STEM with engaging text and visuals that align to Next Generation Science Standards topics. Through this content, students develop a clearer understanding and appreciation for science concepts. Ladders Science includes three reading levels for each of the 36 titles. Students whose reading levels range from 2nd grade to 6th grade will appreciate the articles and introduction to National Geographic Explorers.