Please help.

Problem 3. (1 point) A bacteria culture starts with 640 bacteria and grows at a rate proportional to its size. After 3 hours there will be 1920 bacteria. (a) Express the population after r hours as a function of r. population: (function of () (b) What will be the population after 9 hours? (c) How long will it take for the population to reach 2130 ? Answer(s) submitted: (incorrect) Problem 4. (1 point) An unknown radioactive element decays into non-radioactive sub- stances. In 760 days the radioactivity of a sample decreases by 77 percent. (a) What is the half-life of the element? half-life: (days) (b) How long will it take for a sample of 100 mg to decay to 88 me? time needed: (days) Answer(s) submitted: (incorrect)