If one grain of sand weighs 10^-3 gram, it is one thousandths of a gram. A kilogram (kg), is 1000 times the mass of a gram. So, you take the 10 kg and multiply by a conversion factor of 1 kg equaling 1000g. This results in you having 10000 grams. Multiply that by another thousand, seeing as each grain of sand weighs one thousandth of a gram. This should result in you getting 10000000, or 1.0 x 10^7 grains of sand.
