Most of us are familiar with the conversion formula     that converts Celsius temperatures to Fahrenheit.   Since many of us have difficulty doing fractions in our heads, an "alternate" method is taught to American travelers in Europe:

  1. Double the Celsius temperature.
  2. Subtract 10% of that value.
  3. Add 32.

Most of us can do these computations in our head!
For example, let C = 20 degrees.
Doubling it, we get 40.
Ten percent of 40 is 4.
Subtracting 4 from 40 is 36.
Adding 32 yields 68 degree Fahrenheit.

Using the formula     we arrive at the same answer:
     


Why does this work?
This would be a good exercise for an Algebra I class to do when they are studying combining like terms.