The mass of cobalt-60 in a sample is found to have decreased from 0.800g to 0.200g in a period of 10.5 years. From this information, what is the half-life of cobalt?

By one half life mass would reduce from 800g to 400g,by one more half life it would reduce from 400g to 200g. So 10.5 years is equivalent to two half lives.Thus half life of the isotope is 5.25 years.

The rate of decay is proportional to the amount of the isotope remaining.Half life is the time taken for a given sample to get reduced to half .