Agreed... a pretty tricky explanation. I'll give it a shot. 😀

We are told in the passage that the half-life of iodine-131 is 8 days. Currently, the activity is 1 kBq. Now, we want to know what the activity was two weeks IN THE PAST (which is 14 days ago).

The half-life tells us that the activity will DECAY by 50% in 8 days, so if the activity is 1 kBq NOW, then it will be 0.5 kBq in 8 days.

But, we can look at this a little differently ... half-life ALSO tells us how long it takes for the activity to DOUBLE if we go BACKWARDS in time (which we often do in the movies!).

The activity will DOUBLE if we go back 8 days in the past. If the activity is 1 kBq NOW, then it will be 2 kBq 8 days in the PAST.

That means the activity will be doubled AGAIN if we go another 8 days in the past, or 4 kBq.

Wait a minute... that's a total of 16 days in the past, which is pretty stinkin' close to our desired time period of 14 days!

The closest answer is 3.4 kBq, or "a bit less than 4 kBq", which is the correct answer.

Let me know if that helps. 😄