The number sequences describe a hierarchy of students' concepts of number. This research uses two defining cognitive structures of the number sequences--units coordination and the splitting operation--to model middle-grades students' abilities to write linear equations representing the multiplicative relationship between two unknowns. Results indicate that students who have constructed a tacitly nested number sequence (TNS), second in the hierarchy, do not represent multiplicative relationships algebraically. Students who have constructed an advanced tacitly nested number sequence (aTNS), third in the hierarchy, do so inconsistently, and students who have constructed an explicitly nested number sequence (ENS), fourth in the hierarchy, do so consistently. Coordinating three levels of units in activity advantages aTNS and ENS students, while splitting further advantages ENS students' algebraic reasoning. [For the complete proceedings, see ED606556.]