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
Celsius scale is equal to 180/100 or 9/5 degrees on the Fahrenheit scale.

How to convert Celsius temperatures to Fahrenheit
- Multiply the Celsius temperature by 9/5.
- Add 32
^{o} to adjust for the offset in the Fahrenheit scale.
- Example: convert 37
^{o} C to Fahrenheit.

37 * 9/5 = 333/5 = 66.6

66.6 + 32 = 98.6^{o} F

There is a mental math method to convert from Celsius to Fahrenheit. The
ratio of 9/5 is equal to 1.8 and 1.8 is equivalent to 2 - 0.2

How to convert Celsius temperatures to Fahrenheit with mental math.
- Double the Celsius temperature (multiply by 2).
- Take 1/10 of this number (2 * 1/10 = 0.2) and subtract it from the number above.
- Add 32
^{o} to adjust for the offset in the Fahrenheit scale. -
- Example: convert 37
^{o} C to Fahrenheit.

37 * 2 = 74

74 * 1/10 = 7.4

74 - 7.4 = 66.6

66.6 + 32 = 98.6^{o} F

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 Celsius temperatures to Fahrenheit
- Convert -15
^{o} C to Fahrenheit.
- -15 * 9/5 = -135/5 = -27
- -27 + 32 = 5
^{o} F