We started with 300ml of citrate and the patient drank, or was supposed to drink 180ml. 300-180=120ml, is how much is left in the bottle. So the fraction remaining is (what's left)÷(how much there was in the bottle initially)=120/300. This can be reduced in steps: first divide top and bottom by 10 (just take off the last zero): 12/30. 6 goes into 12 and 30 so we divide top and bottom by 6: 2/5. We can't divide any more so 2/5 is the final fraction.