Calculation device for calculating e.g. kilometer cost of e.g. car, has operating system displaying error message for requesting user to enter value of missing parameter by using key of one activating unit to perform calculation

摘要

The device has an operating system (511) activating a test process in order to determine whether a user has entered correct number of parameters for effectuating a desired calculation. The system displays an error message in one of display zones (311,312) of a display unit for requesting a user to enter a value of missing parameter by using corresponding key of one of activating units (414-416) for performing the calculation. An independent claim is also included for a method of calculation.