Question

A certain radioactive isotope has a half-life of one hour. At noon, 100g of the material is present. How much of the original isotope will be present at
3:00 p.m.----1/8
6:00 p.m.-----1/16
10:00 p.m.-----1/32
Are these correct?

Answer 1

At 3 pm it has gone through 3 half lives and that is 2^3 = 8 so there is 1/8 of 100 left = 12.5 grams. You are correct. The others are not correct.
At 6 pm, there are 6 half-lives so 2^6 = 64; therefore, there is 1/64 of the original 100 or 1.56 g remaining.
Etc.