<em>Half-life time</em> of a radioactive substance is the time for half of the substance to decay.
Thus, the amount of the radioactive substance that remains after a number n of half-lives is given by:
Where:
A is the amount that remains of the substance after n half-lives have elapses, and
A₀ is the starting amount of the substance.
In this problem, you have that the half-live for your sample (polonium-210) is 138 days and the number of days elapsed is 330 days. Thus, the number of half-lives elapsed is:
330 days / 138 days = 2.3913
Therefore, the amount of polonium-210 that will be left in 330 days is: