algebra
posted by meghan .
The halflife of plutonium234 is 9 hours. If 40 milligrams is present now, how much will be present in 4 days? (Round your answer to three decimal places.)

algebra 
Steve
after h hours,
40 * (1/2)^(h/9)
That's because for every 9 hours, the amount is multiplied by 1/2
so, after 4 days = 96 hours, we will have
40 * (1/2)^(96/9) = .0246 mg
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