In order to convert Celsius to Fahrenheit, you will need to use this formula: F = 9/5C + 32. Therefore, you will need to solve for C or substitute values and solve in that manner. To find Fahrenheit temperature for -20 degrees Celsius, you can just plug it into the formula. Therefore, F = 9/5 (-20) + 32 and F = 28.4. If you were given 59 degrees Fahrenheit you would get: 59 = 9/5C + 32 and this would give you 27 (5/9) = C, after you have plugged the values into the above formula. When you simplify you, you get C = 15.