The natural logarithm of the pressure at time 0 is $\ln(1) = 0$. At 2500 seconds, the pressure is 0.947, so $\ln(0.947) = -0.0545$. At 5000 seconds, the pressure is 0.895, so $\ln(0.895) = -0.111$. At 7500 seconds, the pressure is 0.848, so $\ln(0.848) = -0.165$.
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