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Question 1123200: A piece of metal is heated to 428 degree C before it is left to cool. The temperature of the metal falls at the rate of 23 degree C per minute for first three minutes, at the rate of 15 degree C per minute for the next 15 minutes and then at the rate of 8 degree C per minute until it reaches to room temperature of 25 degree C from 428 degree C
i. Find Temperature of the metal after 9 minutes.
ii. The total time needed for the metal to reach a temperature of 25 degree C from 428 degree C.
Answer by Boreal(15235)   (Show Source):
You can put this solution on YOUR website! Temperature drops 23C*3 or 69C in 3 minutes
15C per minute for the next 15 minutes, so that would be 90C in 6minutes for a total drop of 159C in 9 minutes.
i. 428-159=269 C
ii 3 minutes, drops 69 C, 15 minutes, drops 225 C, leaving 134 C as temp after 18 minutes. It is 109 C above 25 C, and it falls 8 C a minute or 109/8 = 13 5/8 minutes more
This requires 31 5/8 minutes or 31m37.5s
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