The average cooling rate of a cup of coffee is 24 °C per minute, and you can begin to sip the coffee when it is 60 °C. How long must you wait before you can begin to sip the coffee if the initial coffee temperature is 156 °C?

Answer 1

Maybe forever because I don't know how you get the temperature to 156 C because all of the fluid would boil away at just over 100 C. But we'll not worry about that; I assume this is just a made up problem and the author of the problem didn't think about that. So you want to cool it from 156 to 60 which is 156-60 = 96C
96C x (1 min/24C) = ? min.