The 1/2 life of Na-24 is 14.9h A facility buys 50mg. How much time will elapse until 12.5% of original amount remains.  I got t=72 hours but it doesn't seem right 

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Hi Rae - Na-24 decays to 1/2 of its original amount after 14.9 hours.  After another 14.9 hours it will again decay 1/2 or 1/2 x 1/2 or 1/4 of the original amount.  Finally, after another 14.9 hours it will again decay by 1/2 or 1/4 x 1/2 = 1/8 (12.5%) of the original amount.  So in total, Na-25 decays to 12.5% of its original amount over 14.9 x 3 hours. In terms of a decay formula, you would solve for t that produces a value of 1/8: 1/8 = (1/2) ^ (t/14.9) This again results in t = 14.9 hours x 3 Hope this helps