Answer:13hrs
Step-by-step explanation:
Initial Temperature = 21°c
final temperature = -18°c
rate of decrease = 3°c/hr
for every hour the temperature decreases by 3°c.
What time does it take to get to 0c = . 21(°c) / 3(°c/hr) = . 7hr
what time does it take to get to -18c = 18(°c) / 3(°c/hr) = . 6hr
Total Time taken = . 7hr + 6hr = 13hr
Alternative solution
Initial Temperature = 21°c
final temperature = -18°c
rate of decrease = 3°c/hr
(21 + 18)°c/ 3(°c/hr) . = . 13hr
