A half-life is how long it takes for half of the amount to go away. In this case, we see that we have two half-life periods worth, which you can determine by dividing the total time by the half-life time.
32hrs/8hrs = 4 half lives.
Now we can raise 1/2 to the power of how many half lives we have (2). Then we multiply that by the amount in the sample.
When you dilate a side, point, etc, you are multiplying the original dimension by the scale factor. That is represented in option C: the original x and y are multiplied by the scale factor, 1/2! Hope that helps! :)
