The boy threw a stone into the well. If the depth of the well is 15 meters, determine the time that elapses from the moment the stone is dropped, to the time the boy hears the sound of the stone in the water. The speed of sound in the air is 340 m/s.

This is for my 8th grade science class, i really don’t understand. can someone me? , lol.

The magnitude of the amount of energy released by burning a fuel source, measured in energy per unit mass, is called its fuel value. note that the fuel value is the negative of the isobaric specific heat of combustion for the fuel. if all the energy obtained from burning 1.23 pounds of butane with a fuel value of 10.85 kcal/g is used to heat 128.0 kg of water at an initial temperature of 18.3 °c, what is the final temperature? note that 1 lb = 453.6 g.

Give one example to show that when substances are mixed, it may result in a chemical or a physical change depending on the conditions.