Given the stream of consciousness rationale, provide a reasonable question and answer. Rationale: In one hour, Addison mountain's temperature will decrease to 3 / 4 of the temperature it is now, meaning the temperature will decrease to 3 / 4 * 84 = 63 degrees. The temperature decrease will be 84 - 63 = 21 degrees.
 The question and answer:
In one hour, Addison mountain's temperature will decrease to 3/4 of its temperature. If the current temperature of the mountain is 84 degrees, what will the temperature decrease by?
The answer is 21