3. A population triples every 10 years. Presently the population is 20,000. a) Find the population in 25 years. b) When will the population be 200,000? c) When was the population 2,000? 4. A radioactive substance decays at a rate of 30% per year. Presently there are 400 g. a) Find the amount left after 8 years. b) How long before there are 10 g left? 5. A radioactive substance has a half-life of 80 years. a) What percent of a same of the substance would be left after 200 years? b) How long will it take before 30% of the sample remains? c) How long will it take before 60% of the sample decays? 6. A radioactive substance has a half-life of 120 years. Presently there are 60 g. a) How much was present 500 years ago? b) How long will it take before 40 g remains? c) When were 600 g present? 7. A substance decays at a certain rate every 4 years. Presently there are 80 g present and 80 years ago there were 300 g present. Determine the decay rate per every 4 years. 8. A population grows 5% per year. a) How long will it take this population to double? b) How long will it take this population to triple? 9. A population doubles every 15 years. Determine the yearly growth rate. 10. A population doubles every 30 years. Determine the yearly growth rate. 11. A radioactive substance decays from 400 g to 120 g in 320 years. Determine its half-life. 12. A radioactive substance decays from 800 g to 50 g in 20 days. Determine its half-life. 13. Money is invested at an interest rate of 4% compounded annually (the money grows by 4% per year). How long will it take the money to (i) double?, (ii) triple? 14. In 1947, an investor bought Van Gogh's painting Irises for $84,000. In 1987 she sold it for $49 million. What annual rate of interest corresponds to an investment of $84,000 to $49 million in 40 years? 15. Jacques bought a new car for $15,000. Each year the value of the car depreciates to 70% of its value from the previous year. In how many years will the car be worth only $500? 16. A pan of water is brought to boil and then removed from the heat. Every 5 minutes thereafter the difference between the temperature of the water and the room temperature is reduced by 50%. If the room temperature is 20 degrees Celsius, how many minutes does it take for the temperature of the water to reach 30 degrees