MATHEMATICS Use the value it – to find the surface area of
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a closed cylinder of radius 7 cm and height 30
cm.
O(A) 1567 cm
O(B) 1686 cm
O(C) 1628 cm
O(D) 2356 cm
OCE
Tuin
jiſ
a

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:  

it would be c. 25 units

answer is

b.

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hope you get this one right.