02 - Time for the thermometer reading to drop to within half degree of the air temperature

Problem 02
A thermometer reading 75 degree Fahrenheit is taken out where the temperature is 20 degree Fahrenheit. The thermometer reading is 30 degree Fahrenheit 4 minutes later. Find the time (in minutes) taken for the reading to drop from 75 degree Fahrenheit to within half degree of the air temperature.
 

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