Growth and decay: B(t) = B0 · q^t
Tell linear, quadratic and exponential tables apart, find a doubling time and a half-life with the logarithm, and see where an exponential model stops being realistic.
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Guide
Growth by p % per step multiplies by the growth factor q = 1 + p100; decay by p % has q = 1 − p100. After t steps the amount is B(t) = B0·qt.
A table is exponential when the ratio of neighbouring values is constant. If the differences are constant it is linear; if the second differences are constant it is quadratic.
To find a time, solve qx = b with the logarithm: x = lg b/lg q. Use the calculator and round at the end.
A model only describes reality for a limited time. Always ask whether the predicted values still make sense.