The metric system uses the Celsius scale to measure temperature. However, temperatures
are still measured on the Fahrenheit scale in the U.S.

Water freezes at 0^{o} Celsius and boils at 100^{o} Celsius which is a difference
of 100^{o}. Water freezes at 32^{o} Fahrenheit and boils at 212^{o}
Fahrenheit which is a difference of 180^{o}. Therefore each degree on the
Fahrenheit scale is equal to 100/180 or 5/9 degrees on the Celsius scale.

How to convert Fahrenheit temperatures to Celsius
- Subtract 32
^{o} to adjust for the offset in the Fahrenheit scale.
- Multiply the result by 5/9.
- Example: convert 98.6
^{o} Fahrenheit to Celsius.

98.6 - 32 = 66.6

66.6 * 5/9 = 333/9 = 37^{o} C.

There is a mental math method to approximate the Fahrenheit to Celsius conversion. The
ratio of 5/9 is approximately equal to 0.55555....

How to approximate the conversion of Fahrenheit temperatures to Celsius with mental math.
- Subtract 32
^{o} to adjust for the offset in the Fahrenheit scale.
- Divide the Celsius temperature by 2 (multiply by 0.5).
- Take 1/10 of this number (0.5 * 1/10 = 0.05) and add it from the number above.
- Example: convert 98.6
^{o} F to Celsius.

98.6 - 32 = 66.6

66.6 * 1/2 = 33.3

33.3 * 1/10 = 3.3

33.3 + 3.3 = 36.6 which is an approximation of the Celsius temperature

Note: This lesson includes temperatures that use negative numbers. Negative
numbers follow a pattern that is opposite of positive numbers. The number -20 is
smaller than the number -10. Example of counting with negative and positive numbers:
-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5

Example of converting negative Fahrenheit temperatures to Celsius
- Convert -4
^{o} F to Celsius.
- -4 - 32 = -36
- -36 * 5/9 = -180/9 = -20
^{o} C