What is Boolean algebra

Which of these is most responsible for differences between the twentieth century to the twenty-first century?

Whats three fourths of 15(this is supposed to be in math but i clicked too fast)

Amitha writes up a one-page summary of a novel during her summer internship at a publishing company. when she reads over the page, she realizes she used the word “foreshadow” seven times, and she would like to reduce the repetition. which tool would best amitha solve this problem?