Question

The half life of Carbon 14 is 5500 years. How many years will pass for a 30 gram sample to reach 3.75 grams

Answer 1

Half life is the time taken by a radioactive isotope to decay by half of its original mass. In this case the half life of carbon-14 is 5500 years. 
The original mass is 30 g and the new mass is 3.75 g. 
New mass = Original mass × (1/2)^n, where n is the number of half lives; 
3.75 = 30 × (1/2)^n
 1/8 = (1/2)^n
 n = 3 
Thus, the time taken will be; 3 × 5500 = 16500 years.