Ans: 15Expln: According to Weber's law, the just noticeable difference (jnd) is a constant proportion. A change from 200 to 220 represents an increase of 20 units and a jnd of 20/200 or 0.10, which is 10 percent. If the company starts with 150 horsepower, they will need to increase it by 10 percent in order for the driver to notice a difference. Ten percent of 150 is 15.