Its very simple. Since 18g of water is a 1 mole of H2O (1*2+16=18) and 1 mole of EVERYTHING is a number of 6,02x10^23 (6,02 and 23 zeros after:) therefore 6,02x10^23 is a number youre looking for firsty. Ok, you say, i have 6,02x10^23 molecules of water, but i want number of oxygen atoms! Thats very easy now, just think : if you have 3 molecules of water, how many atoms of oxygen are there inside ? 3:) Do the same with our bigger number - 6,02x10^23 is the answer:)