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.
36mins/16mins = 2 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.
