In your lab, you measure the amount of a particular radioactive isotope. Six hours later, only 25% of the original isotope is left. How many half-lives have passed?

Question

A) 1

B)2

C)3

D)4

E)5

Answer 1

2 Half lives have passed! After 1 half life there would only be 50% of the isotope left. After another half life, dividing the 50% in half again, we would be left with 25% of the original isotope.

Answer 2

Question is very trivial, but fine, 3 hours = 50%, 6 hours = 25%