Posted by Angie on Wednesday, April 15, 2009 at 11:47pm.
A material has 32 atoms in all; 24 decayed and the rest undecayed. If the halflife of the radioactive material is 1000 years, what is the age of the material?
a. 1000 years
b. 2000 years
c. 3000 years
d. 4000 years

Science  drwls, Thursday, April 16, 2009 at 1:00am
The number of radioactive atoms has decayed to 8/32 = 1/4 of the original value. That takes two half lives.

Science  Angie, Thursday, April 16, 2009 at 1:27pm
Why two halflives?

Science  DrBob222, Thursday, April 16, 2009 at 3:15pm
Angie, you can also work the problem as I showed you on the 30 mCi problem you had earlier.
k = 0.693/1000 years = 6.93 x 10^4
ln(No/N) = kt
[No is the number of atoms we started with; N = number of atoms today]
ln(32/8) = 6.93 x 10^4*t
ln 4 = 6.93 x 10^4*t
1.386/6.93 x 10^4 = t = 2,000 years. 
Science  Angie, Thursday, April 16, 2009 at 3:41pm
ok thanks