newton’s law of cooling states that for a cooling substance with initial temperature t0, the temperature t(t) after t minutes can be modeled by the equation t(t)=ts+(t0−ts)e−kt, where ts is the surrounding temperature and k is the substance’s cooling rate. a liquid substance is heated to 80°c. upon being removed from the heat, it cools to 60°c in 15 min. what is the substance’s cooling rate when the surrounding air temperature is 50°c?