You have 180g of a radioactive substance. it has a half-life of 265 years answer

What mass will remain after 1325 years?
 Radioactive substances are those substances whose unstable atoms undergo spontaneous decay by emitting alpha, beta, or gamma radiations. Half life is the time taken by a radioactive isotope to decay by half its original mass. 
In this case, half life is 265 years,
New mass = original mass × (1/2)∧n where n is the number of half lives,
n = 1325/ 265 = 5
hence, new mass = 180 × (1/2)∧5
                             = 180 × 1/32
                             = 5.625 g
Therefore; a mass of 5.625 g will remain after 1325 years