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I have a relatively simple question. I'm just wondering, with half-life problems, how could I solve something like the following:

Isotope:

**226Ra**

Half-life:

**1620 years.**

Initial Quantity:

**X (Solve for this)**

Amount after 1000 years:

**1.5g**

Where I'm supposed to find the initial quantity.

Wouldn't a part of the equation be something like:

**.5 = e^1620r**, where

**r**is the rate of decay? I'm using natural log to try to get the rate:

**( ln(.5) / 1620 ) = r**

When I punch that in to the calculator, I get huge number and it leads me to believe that I'm messing up somewhere.