7.12 use the concept of half-life to carry out simple calculations on activity

Okay so, this may be a bit confusing but here goes, this is best to show with an example...

E.g. the activity of a radioisotope is 640 Bq. Two hours later it has fallen to 40 Bq. Find the half-life of the sample.

All you have to do is keep dividing 640 until you reach 40... sort of.

Initial Bq is 640

After one half-life 640 / 2 = 320

After two half-lives 320 / 2 = 160

After three half-lives 160 / 2 = 80

After four half-lives 80 / 2 = 40

Okay so we made it to 40 in 4 half-lives, this means it took 4 half-lives for the activity to drop from 640 to 40, which took two hours, meaning 2 hours represents 4 half-lives, meaning each half-life is 30 minutes. So the half-life of this sample is 30 minutes

Answer = 30 minutes.

Example source: CGP