function word problem please help

Question

Strontium 90 is a radioactive material that decays over time. The amount, A, in grams of Strontium 90 remaining in a certain sample can be approximated by the function A(t)=225eˆ(-0.037t), where t is the number of years from now. How many grams of Strontium 90 will be remaining in this sample after 15 years?

I got 391.94 one time i did it and 129.17 the other. please help

Answer 1

Strontium 90 is a radioactive material that decays over time .

The amount, A, in grams of Strontium 90 remains after t years is  A(t)=225 e^(-0.037t) .

So the amount of Strontium 90 existing now ( t = 0 ) is A(0)=225 e^(-0.037*0)

A(0) = 225 e^(0)

A(0) = 225 

So the  amount of Strontium 90 existing now is 225 grams .

Strontium 90 is a radioactive material that decays over time .

So Strontium 90 after 15 years will be less than the value of 225 .

So it never reaches the values greater than 225 ( like 391.94 ) .

The amount of Strontium 90 remains after 15 years is A(15)=225 e^(-0.037*15)

A(15) = 225 (0.5740)

A(7) = 129.17 

So the amount of Strontium 90 remains after 15 years is 129.17 grams .