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The half-life of a radioactive substance is 38.8 years.

a. Find the exponential decay model for this substance.
[tex]\[ A(t) = A_0 e^{-\lambda t} \][/tex]
(Round to the nearest thousandth.)

b. How long will it take a sample of 400 grams to decay to 300 grams?

c. How much of the sample of 400 grams will remain after 10 years?

Answer :

Let's solve the problem step by step.

### a. Exponential Decay Model

The exponential decay model is given by the formula:

[tex]\[ A(t) = A_0 \cdot e^{(-kt)} \][/tex]

where:
- [tex]\( A(t) \)[/tex] is the amount of substance left at time [tex]\( t \)[/tex],
- [tex]\( A_0 \)[/tex] is the initial amount of the substance,
- [tex]\( e \)[/tex] is the base of the natural logarithm,
- [tex]\( k \)[/tex] is the decay constant,
- [tex]\( t \)[/tex] is time.

For a substance with a half-life of 38.8 years, we first need to find the decay constant [tex]\( k \)[/tex]. The formula to find the decay constant when the half-life, [tex]\( T_{1/2} \)[/tex], is known is:

[tex]\[ k = \frac{\ln(2)}{T_{1/2}} \][/tex]

Plugging in the given half-life:

[tex]\[ k = \frac{\ln(2)}{38.8} \approx 0.018 \][/tex]

Therefore, the exponential decay model is:

[tex]\[ A(t) = A_0 \cdot e^{(-0.018t)} \][/tex]

### b. Time to Decay to 300 Grams

We want to find how long it takes for a 400-gram sample to decay to 300 grams. Using the decay formula:

[tex]\[ 300 = 400 \cdot e^{(-0.018t)} \][/tex]

First, divide both sides by 400:

[tex]\[ \frac{300}{400} = e^{(-0.018t)} \][/tex]

[tex]\[ 0.75 = e^{(-0.018t)} \][/tex]

Take the natural logarithm of both sides:

[tex]\[ \ln(0.75) = -0.018t \][/tex]

Solve for [tex]\( t \)[/tex]:

[tex]\[ t = \frac{\ln(0.75)}{-0.018} \approx 16.103 \][/tex]

So, it will take approximately 16.1 years for the sample to decay to 300 grams.

### c. Amount Remaining After 10 Years

To find out how much of the 400-gram sample will remain after 10 years, use the exponential decay formula again:

[tex]\[ A(10) = 400 \cdot e^{(-0.018 \times 10)} \][/tex]

Calculate:

[tex]\[ A(10) \approx 334.561 \][/tex]

So, approximately 334.56 grams of the sample will remain after 10 years.

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