4. 100% 50% 25% Each half life reduces the mass of the sample by half.
Calculate the half life of a substance if 12.5 % of it remains after 90 years?
100%
50%
25%
12.5%
Half life #1
Half life #2
Half life #3
Each half life reduces the mass of the sample by half.
To have 12.5% remain, the original must have been halved three times, three half lives have elapsed.
If three half lives have elapsed within 90 years then each half life must be 30 years.

5. 20 g 10 g 5 g Each half life reduces the mass of the sample by half.
Calculate the time elapsed for and isotope to decay from 20 grams to 2.5 grams. The half life of the substance is 8 years.
20 g
10 g
5 g
2.5 g
Half life #1
Half life #2
Half life #3
Each half life reduces the mass of the sample by half.
To have 2.5 g remain, the original must have been halved three times, three half lives have elapsed.
If three half lives have elapsed, if each is 8 years then the time elapsed is 24 years.

6. 1 0.5 Each half life reduces the mass of the sample by half.
What fraction of an isotope remains after 30 years, the half life is 10 years.
Each half life reduces the mass of the sample by half.
If each half life is 10 years and 30 years have elapsed then there were three half life intervals.
After halving the original ( 1 is the entire original sample) three times, remains. 1
0.5
0.25
0.125
Half life #1
Half life #2
Half life #3

7. 24 12 6 Each half life reduces the mass of the sample by half.
if 3 grams of a radioisotope remains after 30 years, how much was in the original sample? The half life is 10 years. 24 12 6 3
Half life #1
Half life #2
Half life #3
Each half life reduces the mass of the sample by half.
If each half life is 10 years and 30 years have elapsed then there were three half life intervals.
In this example, we double the remaining quantity three times to get the original