Sodium and Potassium

Sodium is EXtracellular, Potassium INtracellular (SEX, PIN)

What are normal values of Sodium and Potassium in serum ? They are 135 - 145 and 3.5 - 5.5 respectively. I used to get confused between these values when I first heard them. This is how I remembered it: Sodium is EXTRAcellular - has EXTRA values, so the larger value is for it - that is 1-3-5, the first three odd numbers and for the smaller potassium, take only second & third odd numbers i.e. 3-5, make it to 3.5