If the 10 mark on your pin equalled 3 ius, 1ml of water would have 30ius(3x10) in it. That would mean all 10mgs of growth would be suspended in 1ml of water, but it's not!
The 10mgs of growth in the cartridge is suspended in 1.5mls of water. So when you divide 30ius of hgh by 1.5mls (150ius total volume of water) the total is 5.
You have 1iu of hgh for every 5ius of water that is in the cartridge.