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First, to find the number of technology books in each category, you need to multiply the percentages or fractions by the total numbers of books sold for each respective category of science/nature books. Then add all those together to find the correct answer. For new adult: 1/3 × 150 = 50. For used adult: 0.50 × 70 = 35. For new children’s: 1/10 × 120 = 12. For used children’s, 1/10 × 120 = 12. The final answer is 50 + 35 + 12 + 12 = 109.