18 is the molar mass of water (2 x 1 + 16 = 18) H2O, H=2g/mol, O=18g/mol
Glucose is different (CxHyOz). Use C=12g/mol, H=1g/mol, O=16g/mol). These values are near enough, but not exact. Use a periodic table if you need the values to more accurate figures.
There are 100mL in 1dL, and 100mL in 1L
It's a trick. The MW of glucose is 180 Da, but the change from liter to deciliter throws in a factor of 10, so the composite factor is 18. (I was confused by it in a previous note and had to work my way through the numbers.)