Sr-90 has a half-life of 29.1 years. How long will it take for the activity of a Sr-90 sample to decay from 6.5x10^3 dpm(disintergration per minute) to 3x10^2 dpm? The half life of a radioisotope is 3 d, how long will it take for a sample of the isotope to decay to 10% of its originalquality?