Fahrenheit and Celsius scales are the most popular scales for temperature measurement. Fahrenheit scale is used only across the USA at this moment. In such countries like Australia, Canada, New Zealand, European countries Celsius scale is used but sometimes old generation people measure temperature with the help of Fahrenheit scale. Almost in all other countries only Celsius scale is used (excluding scientific computation where it is common practice to use Kelvin temperature).
But it is usual in most countries to use Celsius scale for ease of understanding. It is determined by change points of state of water: by 0 degree Celsius water freezes, by 100 degree Celsius water begins to boil; it was the ground for invention of Celsius scale of temperature measurement.
So that to convert Fahrenheit temperature into Celsius temperature it is necessary to subtract 32 from the initial value and multiply the result by 5/9.
So that to convert Celsius temperature into Fahrenheit temperature it is necessary to multiply the initial value by 9/5 and then add 32.
You can use the following technique for quick estimated conversion of Fahrenheit temperature into Celsius temperature "mentally": subtract 32 from the initial value, divide by 2 and add one tenth of this value.
For instance, 72 °F approximates (72 - 32) / 2 = 20 plus 1/10 from 20, that is 22 °C.