$N=N_0e^{kt}$ where N is the amount after decay, No is the initial amount, e is Euler's number, k is the decay constant, and t is the time in minutes. For us, the first equation looks like this:

$7=12e^{k(70)}$ and we will solve that for k and then use that value of k in the second equation to find time.

Begin by dividing 7 by 12 to get

$\frac{7}{12}=e^{k(70)}$ Now take the natural log of both sides since the natural log and that e are inverses. The e then disappears:

$ln(\frac{7}{12})=k(70)$

Plug the left side into your calculator and then set it equal to the right side, giving us:

$-.5389965007=70k$

Divide both sides by 70 to get the k value of

k = -.00769995

Now the second equation looks like this:

$2=12e^{(-.00769995)t$ where t is our only unknown now that we know k.

Begin by dividing both sides by 12 to get

$\frac{1}{6}=e^{-.00769995t$ and take the natural log of both sides again to eliminate the e:

$ln(\frac{1}{6})=-.00769995t$

Take care of the left side on your calculator to get

-1.791759469 = -.00769995t and divide both sides by -.00769995 to get

t = 232.697 minutes

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3 years ago