Example 1 Continued

Step 3. Write a model for the exponential decay.

Remember that the general formula for an exponential function is RealWorldEx1-1

a is the initial value, so a = 20.
is the common ratio, so b = 1/2.

Therefore, the equation is RealWorldEx1-2

Step 4. Determine the amount of radioactivity left after three weeks.

Since x represents the number of weeks, let x = 3.