2007 AIME I Problems/Problem 5
The formula for converting a Fahrenheit temperature to the corresponding Celsius temperature is An integer Fahrenheit temperature is converted to Celsius, rounded to the nearest integer, converted back to Fahrenheit, and again rounded to the nearest integer.
For how many integer Fahrenheit temperatures between 32 and 1000 inclusive does the original temperature equal the final temperature?
Examine modulo 9.
- If , then we can define . This shows that . This case works.
- If , then we can define . This shows that . So this case doesn't work.
Generalizing this, we define that . Thus, . We need to find all values that . Testing every value of shows that , so of every values of work.
There are cycles of , giving numbers that work. Of the remaining numbers from onwards, work, giving us as the solution.
Notice that holds if for some . Thus, after translating from we want count how many values of there are such that is an integer from to . This value is computed as , adding in the extra solution corresponding to .
Let be a degree Celsius, and rounded to the nearest integer. Since was rounded to the nearest integer we have , which is equivalent to if we multiply by . Therefore, it must round to because so is the closest integer. Therefore there is one solution per degree celcius in the range from to , meaning there are solutions.
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