Question

A sample of a radioactive substance is reduced to 1.0 gram after 4 half-lives. Determine the mass of the original sample

before it decayed.
A 1/16 grams
B 16 grams
C 8 grams
D 1/8 grams

Answer 1

After 4 half-lives, there was 1 gram.

1 half-life earlier, after 3 half-lives, there must have been 2 grams.

1 half-life earlier, after 2 half-lives, there must have been 4 grams.

1 half-life earlier, after 1 half-life, there must have been 8 grams.

1 half-life earlier, when it started decaying, there must have been 16 grams. (B)