Celsius to Fahrenheit without a calculator
The exact formula is Fahrenheit = Celsius × 9/5 + 32. But when you are travelling and have no calculator, a quick estimate is usually enough.
The shortcut: double the Celsius number, subtract ten percent of the result, then add 32. For 20 °C: double it to 40, subtract 4 to get 36, add 32 to get 68 °F — exactly right.
It works because doubling is close to multiplying by 1.8, and the ten-percent correction fixes most of the gap. For rough weather sense, you can go simpler still: 0 °C is freezing, 10 °C is cool, 20 °C is pleasant, 30 °C is hot, 40 °C is a heatwave.
For exact work — cooking, science, medicine — use a converter. For the street, the doubling trick will get you within a degree or two, which is all a jacket decision needs.