PE = mgh, solve for m

Astudent carefully transfers 30 g of water and 30 g of alcohol in a glass tube, forming two layers and filling the tube completely. after sealing the tube, the student mixes the solutions, and notices a bubble that forms in the tube. what is the mass of the contents in the glass tube after mixing?

Why does his teacher ask him to balance the equation by including the correct coefficient

How many neutrons does an element have if its atomic number is 50 and its mass number is 166