(a) If time t = 0, Theta = 80. Find A. Obviously...80.

(b) If the temperature of the surroundings is 20 degrees Celsius and the body cools to 70 degrees Celsius in 10 minutes, find:

**(i) The body's temperature after 20 minutes.**

(ii) The time taken to reach 60 degrees Celsius.

(ii) The time taken to reach 60 degrees Celsius.

21. A radioactive substance decays at a rate that is proportional to the mass of radioactive substance present to any time. If 10% decays in 200 years, what percentage of the original radioactive mass will remain after 1000 years?

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