A"principal square root" is a: a negative fraction b: a positive integer c: a negative integer d: a positive fraction

Really none of the above.  

A principal square root of a real number is a positive real number (or a positive real number times i, if you're up to complex numbers, but let's assume not.)  It's not necessarily a positive integer, √2 being the obvious example.

Is √2 a positive fraction?   We can write it as √2 / 1 so I suppose it is.  It's certainly not a rational number.  Even if we grant irrationals as fractions surely √0=0 isn't a positive fraction.  So "always" isn't correct.

I'd go with none of the above, but if I had to choose I'd choose D.

ending amount = beginning amount / (2^n)

where "n" = number of half-lives (or elapsed time / half-life)

ending amount = 50 mg / (2 ^10 / 5.3)

ending amount = 50 mg / (2 ^ 1.8867924528)

ending amount = 50 m / 3.6981210597

ending amount = 13.5203794557 mg

or about 13.5 mg remain

source:  

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