Posted by brittany on Sunday, May 15, 2011 at 11:58pm.
100 - 12.2 = 87.8%
dx/dt = -.122 x
dx/x = -.122 dt
ln x = -.122 t + C
x = e^(C-.122 t)
x = c e^(-.122 t)
when t = 0, x = 100%
so
x = 100 % e^(-.122 t)
(100 %-31 %) = 100 % e^-.122 t
.69 = e^(-.122 t)
ln(.69) = -.122 t
-.371 = -.122 t
t = 3.04 years
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