At dawn the temperature is 4 degrees Celsius. By midnight it has fallen by 9 degrees. Enter the midnight temperature in degrees Celsius.

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A fall of 9 uses up the 4 degrees that stood above zero and then carries the reading 5 degrees further past it, ending 5 degrees below zero. Answering 5 keeps the distance from zero but places the reading on the warm side, and adding the fall to the start reverses the direction of the change.