I. From the end of the 5th chapter (out of 27 chapters) of Wentworth's New School Algebra (published in 1898), pp. 70-71 [click to enlarge]:
II. From the end of the 5th chapter (out of approx. 16 chapters) of the Singapore Math Discovering Mathematics 7A (Common Core), p. 29) [click to enlarge]:
My daughter finished up the Singapore Math Challenging Word Problems for 6th grade back in October and, after checking out the two Singapore Math sequels, New Elementary Mathematics and Discovering Mathematics, I opted instead to start her on Wentworth's New School Algebra. She's no math genius, but, after all that solid 1st-6th grade Singapore Math, complete with the Challenging World Problems series, I felt she was ready to move beyond arithmetic straight into algebra--something that neither the Singapore Math sequels do. There is some algebra in New Elementary Mathematics and Discovering Mathematics, but there's still a lot of arithmetic mixed in.
Wentworth's 1898 New School Algebra, on the other hand, moves straight into algebra, in the most straight forward, systematic way I've seen in any algebra book. Chapter I: Definitions and Notation; Chapter 2: Simple Equations; Chapter Three: Positive and Negative Numbers [in algebraic expressions]; Chapter Four: Addition and Subtraction [of polynomials]; Chapter Five: Multiplication and Division [of polynomials]. So straight forward and systematic is Wentworth's curriculum that my 6th grade daughter, after doing every single problem leading up to this, is now working on the above problem set with minimal assistance from her mother.
It makes me wonder how many other 6th graders could get this far with algebra, if only they were given a Singaporean foundation in arithmetic followed by a Wentworthian systematicity in introductory algebra.