AlejandroDeLaPreCal wrote:This exercise uses the radioactive decay model.

To see how to do this, try

this page.

AlejandroDeLaPreCal wrote:The half-life of cesium-137 is 30 years. Suppose we have a 128-g sample.

So P = 128.

AlejandroDeLaPreCal wrote:(a) Find a function that models the mass remaining after t years. (Round rate r to three significant figures.)

What is half of 128? This is A. This is how much you have at t = 30. Plug these into the equation (A = Pe^{rt}) and solve for r.

AlejandroDeLaPreCal wrote:(b) How much of the sample will remain after 120 years? (Round your answer to one decimal place.)

Use the P they gave you, the r you just found, and the t they give you for this part. Plug them into the equation and solve for A.

AlejandroDeLaPreCal wrote:(c) After how long will only 4 g of the sample remain? (Round your answer to the nearest whole number.)

Use the P they gave you, the r you found, and the A they give you for this part. Plug them in and solve for t.

If you get stuck, please write back showing what you've done. Thanks.