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# At time zero, there are 10.0 grams of W-187. If the half-life is 23.9 hours, how much will be present at the end of seven days?

$\begin{array}{1 1} 0.1428\;g \\ 1.428\;g \\ 0.078\;g \\ 0.78 \;g \end{array}$

23.9 hours = 0.9958 of one day. Let us assume for sake of simplicity that half-life = 1 day = 24 hours.
(starting amount) x (1/2)$^{\text{number of half-lives}}$ = ending amount (sometimes remaining rather than ending is used)
starting amount = 10.0 g
Seven days = 7 half-lives; (1/2)$^\text{7}$ = 0.0078 remaining = 0.078 g.
edited Apr 2, 2014