#68 and #70 please

68. T Radiouctive Decay The half-life of plutonium-239 is 24,400 years. How long, to the nearest year, do you expect it to take for a randomly selected plutonium-239 atom to decay? 69. V The Doomsday Meteor The probability that a "doomsday meteor" will hit the Earth in any given year and release a billion megatons or more of energy is on the order of 0.00000001.16 When do you expect the Earth to be hit by a doomsday meteor? (Use an exponential distribution with a = 0.00000001.) 70. Galactic Cataclysm The probability that the galaxy MX-47 will explode within the next million years is estimated to be 0.0003. When do you expect MX-47 to explode? (Use an expo- nential distribution with a = 0.0003.)