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- Feb 25th 2010, 11:35 AM #1

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## help with a question

the active chemical in a certain medicine breaks down according to an exponential law. if the chemical has a half life of 20 years find the shelf life of the medicine if it is considered to be expired when less than 90% of the active chemical is still available...

pls can someone show me how to go about these type of questions as i have many similar questions to go through like this...

- Feb 25th 2010, 11:46 AM #2

- Feb 25th 2010, 11:56 AM #3

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- Feb 25th 2010, 12:12 PM #4

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you use the 1/2 as your base because its a half life, and then it is to the x power, but since the half life only occurs every 20 years the x is over 20.

Now to find when there is less than 90% of the chemical you simply set the equation equal to .1, aka 10%, and solve.

- Feb 25th 2010, 12:20 PM #5

- Feb 25th 2010, 01:10 PM #6

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hi there thanks for the help with that question....are these the type of questions that logs are needed for?? bcuz i got a question like in 1990 west germany population was 60 million and decreasing at rate of 0.1% per year, and east germany population was 16 million and increasing at 0.06% per yr...if this continued when would the populations be equal? my friend tells me the answer is definately 2816AD but he uses a really complex formuale. is it possible to work this out with logs as they seem really handy. thank you

- Feb 26th 2010, 12:25 AM #7

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hi there thanks for the help with that question....are these the type of questions that logs are needed for?? bcuz i got a question like in 1990 west germany population was 60 million and decreasing at rate of 0.1% per year, and east germany population was 16 million and increasing at 0.06% per yr...if this continued when would the populations be equal? my friend tells me the answer is definately 2816AD but he uses a really complex formuale. is it possible to work this out with logs as they seem really handy. thank you