Chromium-51 is a radioactive isotope used to label red blood cells to identify if they remain intact in the body. It has a half-life of 27.7 days. If 0.25 mg is an approximate dose amount, how much remains after 83.1 days?

Since the half life is the the time taken for half of the Chromium-51 to decay, for n half lives, the mass of Chromium-51 will fall to 1 / (2^n) of its original value. 83.1/27.7 = 3 so the mass of Chromium-51 will fall by a factor of 1 / (2^3) = 1/8.

0.25 mg / 8 = 0.031 mg (2 significant figures).