I missed a class and I don't know how to do this problem help!?

Question

1) The first atomic explosion was detonated in the desert north of Alamogordo, New Mexico, on July 16, 1945. What fraction of Strontium-90(t1/2=28.8 years) originally produced by the explosion still remains as of July 16, 2008?

2) A rock sample contains 627 atoms of uranium-235 for each 10,000 atoms of lead-207. The half-life of U-235 is 7.0x10^8 years. how old is the rock? (Assume that all lead-207 originated from the decay of U-235.)

Answer 1

These aren't as hard as they look. Read up on "half life" and there should be an equation.

Hints:

1) For instance in 28.8 years only half of the Strontium-90 would remain. In 57.6 years only 1/4 would be left. It's been 63 years, how much is left?

2) This is like the first problem but in reverse.

Answer 2

you want us to do your home work! lol