In the video about half-life, ra-223 decays to rn-219 by alpha decay with a half-life of 11 days. assuming you start with 160 grams of ra-223, how much would be left after 33 days?

Radioactivity or radioactive decay is the process by which unstable isotopes or atoms release energy inform of radiations such as alpha, beta and gamma radiations, to attain stability. Half life is the time taken by a radioactive substance to decrease by half its original mass. 
New mass = Original mass × (1/2)^n where n is the number of half lives.
n in this case is, 33/11 = 3
Therefore, the new mass = 160 × (1/2)^3
                                         = 160 × 1/8
                                         = 20 g
Thus, the mass left after 33 days will be 20 g

The answer to this problem is. 67