Question

How many half-lives does it take a radioactive substance to decay until only one-eighth of the original substance remains?(1 point)
Responses

eight
eight

three
three

two
two

six
six
Question 5
The half-life for a 100-gram sample of radioactive element X is 5 days. How much of element X remains after 10 days have passed?[This is the stem.](1 point)
Responses

25 g
25 g

50 g
50 g

75 g
75 g

100 g

Answer 1

1) three — because (1/2)^3 = 1/8.

2) 25 g — 10 days = 2 half-lives, 100*(1/2)^2 = 25 g.