1.

Approximately what percentage of parent isotopes remains
after 2 halflives have passed? 

50%
25 %
6.25%
2%
0.39% 
2. 
If a rock initially contained 10 milligrams of a radioactive
parent when it first crystallized, how much remains after 4 halflives? 

4 mg
2.5 mg
0.625 mg
.05 mg
Unknown 
3. 
What % parent remains after 10 halflives? 

10%
1%
0.1%
0.01%
Unknown 
4. 
If a mineral contains 1.56% of its original parent isotopes,
how many halflives have passed? 

6
7
8
9
1.56 