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  about 81 years

Step-by-step explanation:

A graphing calculator can give you the answer easily.

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You can solve the problem algebraically by putting in the given number and solving for t.

  30000 = 32000/(1 +12.8e^(-.065t))

  12.8e^(-.065t) = (32000/30000) -1 = 1/15

  e^(-.065t) = 1/(15·12.8) = 1/192 . . . . . divide by 12.8

  -0.065t = ln(1/192) ≈ -5.257 . . . . . .  take the natural log

  t = -5.257/-0.065 ≈ 80.88 . . . . . . . .divide by the coefficient of t

It will take about 81 years for the number of trees to reach 30,000.