16. A radioactive isotope has a half-life of 13 hours. Find the amount of the isotope left from a 400-milligram sample after 65 hours. If necessary, round your answer to the nearest thousandth.
a. 25 mg
c. 0.049 mg
b. 7.692 mg
d. 12.5 mg
Please help. I only see answers for 52 hours

Without the math: 65 hours is 5 half-lives

13 hrs = 1/2

26 hrs = 1/4

39 hrs = 1/8

65 hrs = 1/32

(1/2)^5 = 1/32

400 mg / 32 = 12.5 mg   that is, after 5 half-lives

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