Choose the equation that is a linear function. A
1
4
x +
3
4
y = 16
B
3x + 3 = 3y
C
both A and B are linear functions.
D
neither A nor B are linear functions.

one fourth of 9% = 2.25%

gavin is correct in saying : the equivalent quarterly interest rate would be

2.25%.

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:  

is there any other information given for this problem? this is not enough information.