A 78 gram sample of uranium loses half of its mass each year. how much is left after 6 years?

After six years, 1.21875 gram of uranium will remain, since uranium loses half of its mass each year. At end of year 1, 39 gram is left. At end of year 2, 19.5 gram; at end of year 3, 9.75 gram; at end of year 4, 4.875 gram; at end of year 5, 2.4375 gram and finally, at end of year 6, 1.21875 gram is left.

1 year is the half life of Uranium that is time taken for uranium of specific isotope to fall to half its original value

After 6 years the mass of uranium will be

Final mass = intial mass x (1/2) ^time taken

that is 78 x (1/2) ^6=1.219grams