Hi Matthew,

I am as confused by this problem as you are. Is this from a text book?

I don't know what the units are but you might say that the child's sodium level is 29 units below the bottom of the normal range. To express this as a percentage you need to say this is so many percent of "something" and I don't see any quantity to substitute for "something" to make a percentage meaningful.

I suppose you could look at this as if you were pumping up a tire and the normal range for the pressure you want is between 136 and 145 units. You started with a pressure of $0$ units and now your pressure gauge reads $107$ units so you are $\frac{107}{136} \times 100 = 79\%$ of the way to the bottom of the normal range leaving you $21\%$ of the way to go.

I hope this helps,
Harley