The radioactive substance neptunium-139 decays to 73.64% of its original amount after 24 hours. How long (to the nearest hour) would it take for 39% of the original neptunium to be present?
?
A. 83 hr
B. 77 hr
C. 39 hr
D. 74 hr
E. 68 hr
Answer: D
Mathematics
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A. 
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