Question 1123200
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