Before we go on to discuss
exponential decay, we should pause and discuss some of the terms that are
frequently used in these problems. First let's talk about the term **factor**.

Suppose you invested $100, and after a time, your investment was worth
$300. The final value ($300) would be three (3) times the initial value.
We would say that your investment had *increased by a factor of 3*.

On the other hand, if you made a poor investment, and the value decreased
from $100 to $25, then the final value would be a quarter (1/4) of the
initial value. We would say the investment had *decreased by a factor
of 4*.

If a population of 500 *increased* by a factor of 1.5, then there
would be a population of (500)(1.5) = 750. If the same population *decreased*
from 500 by a factor of 1.5, then there would be 500/1.5 = 333 remaining.

When there is an **increase**, the ratio of the final number to the
initial number is the **factor**. That is

Percentage Changes

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