A 12.65 g sample of a radioactive substance is allowed to decay for 17.22 min. At that time, the sample weighed 3.115 g. What is the half-life for this substance?

8.61 min

Explanation:

original mass = 12.65

first half life = 12.65/2 = 6.325

second half life = 6.325/2 = 3.1625

Note : 3.1625 is the closest to the value (3.115) given so we work with it

total time for decay = 17.22

therefore two decays = 17.22/2 = 8.61