Abstract: A computing system is disclosed which operates using a digital computer which is programmed with a known programme, and information is fed into the computer from an input unit which incorporates a means to apply specific values for the initial conditions of selected variables and further means which are manually actuable to vary the value of the quantity representing a designated variable from its initial condition during computation in order that the response to such variation may be monitored on a display unit which conveniently is in the form of a video display unit. Additionally monitoring means in the form of lights which are operable in response to constraint violation may be included. The manually actuable means is disclosed in a form which is electronic.