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The function [tex]A = A_0 e^{-0.00693 x}[/tex] models the amount in pounds of a particular radioactive material stored in a concrete vault, where [tex]x[/tex] is the number of years since the material was put into the vault. If 500 pounds of the material are initially put into the vault, how many pounds will be left after 70 years?

A. 357 lb
B. 308 lb
C. 175 lb
D. 186 lb

Answer :

To solve this problem, we are using the formula for radioactive decay:

[tex]\[ A = A_0 e^{-0.00693x} \][/tex]

where:
- [tex]\( A \)[/tex] is the amount of radioactive material remaining after [tex]\( x \)[/tex] years.
- [tex]\( A_0 \)[/tex] is the initial amount of radioactive material.
- [tex]\( x \)[/tex] is the time in years.
- [tex]\( e \)[/tex] is the base of the natural logarithm, approximately equal to 2.71828.

In this problem, we are given:
- The initial amount [tex]\( A_0 = 500 \)[/tex] pounds.
- The decay rate constant is 0.00693.
- The time [tex]\( x = 70 \)[/tex] years.

Now, we need to find the amount remaining, [tex]\( A \)[/tex], after 70 years.

Step-by-step solution:

1. Start with the initial amount [tex]\( A_0 = 500 \)[/tex] pounds.
2. Substitute [tex]\( x = 70 \)[/tex] years and the decay rate constant into the formula:

[tex]\[ A = 500 \times e^{-0.00693 \times 70} \][/tex]

3. Calculate the exponent:

[tex]\[ -0.00693 \times 70 = -0.4851 \][/tex]

4. Calculate [tex]\( e^{-0.4851} \)[/tex]. This involves using a scientific calculator or mathematical software to find the value of [tex]\( e\)[/tex] raised to the power of [tex]\(-0.4851\)[/tex].

5. Multiply the initial amount by the calculated decay factor:

[tex]\[ A = 500 \times e^{-0.4851} \][/tex]

After performing these calculations, you will find:

[tex]\[ A \approx 307.82 \][/tex] pounds

Therefore, the amount of radioactive material remaining after 70 years is approximately 308 pounds.

The closest answer choice is:

b) 308 lb

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