Step 1 :

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(a)

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The amount $\"\"$ of a radioactive material present at time $\"\"$ is $\"\"$ .

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Where $\"\"$ is the original amount of the radioactive material and $\"\"$ is a negative number that presents a rate of decay.

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The function is $\"\"$ .

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Compare the function with $\"\"$ .

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The decay rate of strontium 90 :

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$\"\"$

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The decay rate of strontium 90 is $\"\"$

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Step 2 :

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(b)

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The function is $\"\"$ .

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Substitute $\"\"$ and $\"\"$ in the function.

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$\"\"$

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After 10 years, 391.74 grams of strontium 90 be left.

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Step 3 :

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(c)

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The function is $\"\"$ .

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Substitute $\"\"$ and $\"\"$ in the function.

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$\"\"$

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After $\"\"$ , 400 grams of strontium 90 be left.

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Step 4 :

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(d)

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The function $\"\"$ .

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Half-life of the function $\"\"$ .

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Substitute $\"\"$ in the function.

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The function is $\"\"$ .

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Solve for $\"\"$ .

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$\"\"$

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Half-life of strontium 90 is $\"\"$