Q2: There is a volume of 1 cm^3 of water, and as we are given the density we know also that this is equal to 1g of water. Using the equation moles = mass/molar mass, use this mass and divide it by the molar mass of water (18gmol^-1). This gives you the amount of moles of water. Now as we know that 1 mole has Avagadro's Constant (6.022x10^23) molecules, so we multiply the moles of water by Avagadro's constant.
Q3 is the same principle, except the density is not the same as the volume, so mass will be different.