That MCAT will not know what hit it!

For this questions think back to Newton's third law. For every action there is an equal and opposite reaction. If we were to place the spring vertically on the floor and put a weight on the spring the weight would apply a force the spring and do would the floor. For this problem the force is 100N and the spring is deformed by an amount of 10cm + 10 cm = 20 cm.