exponential decay , word problem

Exponential decay: The amount of a radioactive material present at time t decays at a rate proportional

to the amount present at that time. The half life h is the time at which half of the material has decayed.

The mathematical model for exponential decay is

Here y0 is the initial amount of the substance present. After half lives, the material has been successively halved times.

Hint: recall that

Uranium-238 has a half life of years. Let A be an amount of Uranium-238. What fraction of A is the amount left after years?

b. 238 grams of Uranium-238 contain Avogadro's number, approximately atoms. Using this approximation for Avogadro's number, how many atoms of 238 grams of Uranium-238 will decay in the first

year? In the second year?