Abstract: In one aspect, the invention provides computational device that comprises a processor for performing mathematical computations, and a user interface. The user interface comprises a display and a plurality of functional buttons each associated one of a plurality of variables in a mathematical equation defining a relationship among those variables. In response to the selection of any of the buttons by a user, the processor effects presentation of one or more prompts in the display to invite the user to enter values for variables associated with buttons other than that selected by the user, and upon entry of the last variable value, the processor calculates a value for the variable selected by the user and presents said calculated value in the display.

Abstract: In one aspect, the invention provides computational device that comprises a processor for performing mathematical computations, and a user interface. The user interface comprises a display and a plurality of functional buttons each associated one of a plurality of variables in a mathematical equation defining a relationship among those variables. In response to the selection of any of the buttons by a user, the processor effects presentation of one or more prompts in the display to invite the user to enter values for variables associated with buttons other than that selected by the user, and upon entry of the last variable value, the processor calculates a value for the variable selected by the user and presents said calculated value in the display.

Abstract: In one aspect, the invention provides computational device that comprises a processor for performing mathematical computations, and a user interface. The user interface comprises a display and a plurality of functional buttons each associated one of a plurality of variables in a mathematical equation defining a relationship among those variables. In response to the selection of any of the buttons by a user, the processor effects presentation of one or more prompts in the display to invite the user to enter values for variables associated with buttons other than that selected by the user, and upon entry of the last variable value, the processor calculates a value for the variable selected by the user and presents said calculated value in the display.

Abstract: In another aspect, a handheld electronic device is disclosed that includes a body sized and shaped for holding by one hand of a user. The device further includes a user interface comprising a plurality of buttons for operating the device and at least one securing element coupled to the body. The securing element is adapted for engagement with at least one digit of the user's hand to facilitate holding the device so as to allow the user to operate the device, via its user interface, with one or more other digits of that same hand. In some embodiments, the device can be utilized for calculating a variable value associated with a loan (e.g., monthly payment) based on entered values of the other variables of the loan (e.g., amount, interest rate, and duration).