On the surface of earth, a spacecraft has a mass
of 2.00 × 104 kilograms. what is the mass of the
spacecraft at a distance of one earth radius
above earth’s surface?
(1) 5.00 × 10^3 kg (3) 4.90 × 10^4 kg
(2) 2.00 × 10^4 kg (4) 1.96 × 10^5 kg

The radioactive isotope of lead, pb-209, decays at a rate proportional to the amount present at time t and has a half-life of 3.3 hours. if 1 gram of this isotope is present initially, how long will it take for 80% of the lead to decay? (round your answer to two decimal places.)