Answer :
To obtain Dalton's atomic theory regardless of the water source, Water is 2 parts Hydrogen(1*2) and 1 part Oxygen(16). We divide 2/18 for hydrogen and 16/18 for Oxygen to get the atomic mass percentage, knowing that the atomic mass of hydrogen is close to 1 and the atomic mass of Oxygen is close to 16. That is from the second part of Dalton's 4 part theory.