1. A method, implemented at a computer system that includes one or more processors, for solving in the context of a model that includes a plurality of model variables, the method comprising: an act of the computer system displaying a user interface, the user interface including: an equation edit area that displays a plurality of equations that represent analytical relationships between a plurality of model variables; and an output variable edit area that is distinct from and visually distinguished from the equation edit area in the user interface and that is configured to receive user input that specifies a subset of the plurality of model variables as output model variables that are to be solved for within the model using the plurality of equations, wherein the output variable edit area is also configured to allow a user to input different sets of model variables that are to be solved for using the plurality of equations without modifying the plurality of equations in the equation edit area; an act of the computer system receiving first user input at the output variable edit area that specifies a first subset of one or more of the plurality of model variables as output model variables that are to be solved for using the plurality of equations; based on receiving the first user input, an act of the computer system formulating an output variable data structure that identifies the first subset of output model variables; an act of the computer system formulating an equation data structure that represents the analytical relationships between the plurality of model variables of the plurality of equations; based on receiving the first user input, an act of the computer system automatically solving the plurality of equations for the first subset of output model variables using a solver framework; subsequent to solving the plurality of equations, an act of the computer system receiving second user input at the output variable edit area that specifies a second subset of one or more of the plurality of model variables as output model variables, wherein the second subset is different than the first subset; based on receiving the second user input, an act of the computer system modifying the output variable data structure to identify the second subset of output model variables; and based on receiving the second user input, an act of the computer system using the solver framework to automatically solve the plurality of equations for the second subset of output model variables, without modifying the plurality of equations in the equation edit area, wherein the solver framework does not change depending on the identity of the subset of output model variables identified in the output variable data structure.