Can you figure out what is inside the six identical boxes below? The voltage at output z is the same simple function of the x and y input voltages in each case.

The six boxes are chained, all x inputs and one y input being connected to points of fixed potential. The voltages (in millivolts, say) resulting at each input and output are then as shown. With one exception: the point labelled "?". The final output voltage is seven millivolts.

Can you describe the circuitry inside the box?



 

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