The hot water when poured into the glass containing cold tap water.

            The hot water remained at the top when poured into the glass containing cold tap water.

Explanation:
                   As we know every thing which floats on water has less density as compared to water. So, it means when water is heated its density decreases or when it is cooled its density increases. Don't get confused with Ice. Density of water decreases upto 5 to 6 °C, below this temperature Ice formation takes place. 
                  So, when water is heated the molecules of water starts moving with higher kinetic energy. The space between water molecules increases and tends to occupy more space, hence an increase in volume takes place.  As density and volume are inversely related to each other so density decreases and hot water moves to the top.

Ithink it is quantity of plants

Answer is to avoid experimental error