Findings are presented from an analysis of how six future middle-grade teachers reasoned with strip diagrams and a variable parts perspective on proportional relationships to develop and explain equations in two variables. One equation was for two quantities varying together and one was for a line through the origin in a coordinate plane. Both equations involved a constant of proportionality that was not a whole number. The future teachers' arguments were mathematically valid and relied on reasoning quantitatively about strip diagrams. The arguments also treated variables as quantities but rarely described the variables as numbers of units. Some arguments combined an interpretation of fractions as multiples (or iterates) of unit fractions with an interpretation of multiplication as a whole number of equal groups. In contrast, most arguments involving a fractional multiplier interpreted multiplication as "of." All the points of tension that the future teachers encountered while developing their equations concerned referent units for quantities. The points of tension were resolved by focusing on referent units or on equality. Based on the data, extensions to current theories on reasoning quantitatively and with variables are proposed. [For the complete volume, "Mathematics Matters in Education: Essays in Honor of Roger E. Howe. Advances in STEM Education (ASTEME)," see ED607653.]