In a laboratory experiment, protactinium-234 undergoes radioactive decay by β-emission into uranium-234. The table below describes how the mass of uranium-234 present in the sample varies with time from the start of the experiment: Using the information in the table, approximately what is the half-life of protactinium-234?

Answer:

3) A

This may seem slightly confusing.
To start with, we need to know the starting mass of protactinium-234.
We know that all of protactinium-234 undergoes decay. So all starting mass of protactinium-234 must have undergone decay to make uranium-234.

We know the total mass of uranium-234 formed was 16 g. This means that the starting mass of protactinium-234 also must have been 16g. The time it took to decay into half (8g) was 1.2 minutes. Hence A is the answer.

Another way of doing this is to see the difference between the production of uranium-234.
Within the first 1.2 minutes, 8 g was formed. After the second 1.2 minutes, 4g was formed. After the third 1.2 minutes, 2g was formed. From this, we can see that after every 1.2 minutes, the production of uranium-234 becomes half until the maximum amount of uranium-234 is formed.