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When a 4kg mass is hung vertically on a light spring, the spring stretches 2.5cm. A.) How far will the spring stretch if an additional 1.5kg mass is hung on it? B.) If the 4 kg mass is removed, how far will the spring stretch if a 1.5 kg mass is hung on it? C.) How much work must be done on the spring to stretch the same spring 4 cm from its equilibrium position?