what is a half life of a substance and how can you work it out.

A half life of a substance is the time it takes for the mass of a radioactive substance to decrease by half. Say you have 50g of carbon-14, you are looking for the amount of time it takes for the mass to reach 25g of this isotope.
Say we have 200g carbon-14, and it has a hypothetical half-life of 5 years. How much will be left after 20 years?So, we do 20/5=4, so we have 4 half lives undertaken. If a half life is dividing the mass by 2, each progressive half life also decreases the mass by dividing it by 2. So, we can think of dividing our original amount by 2^n, where n is the number of half lives passed. Therefore, we have 200/(2^4) or 200/16, which equals 12.5g remaining of carbon-14.