Hi i'm reviewing for my midterm and I have no clue how to even start this question.. If you could help and show steps PLEASE help! Thank you.

5. The rate of change of the temperature of a cup of tea is 40e_t degrees celsius per hour, t hours after it was made. If the initial temperature was 90" C, (a) What was the temperature 30 minutes later? (b) What was the temperature t hours later? (0) When was the tea at 80 C