The angles of a quadrilateral are
represented by 9xº, (20x + 16)°,
(12x + 10), and (7x - 2)°. what is the
value of x?
a 7
b 9
c 12
d 14

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: