When a mass is suspended from the end of a spring, it lengthens. For the same mass, the lengthening varies according to the spring used. The spring A opposite measures 8 cm. It lengthens. 1 cm for each 100g mass that is suspended at its end. The spring B measures 5 cm and extends by 1 cm for each 75g mass that is suspended at its end. It is possible to suspend the same mass to these two springs so that they are of the same length, once extended. What is this mass?