According to Conversion formula above if you want to convert degrees Celsius to degrees Fahrenheit you have to multiply degrees Celsius by 9, divide the result by 5 and add 32 to it.
So you have: ((28 × 9) ÷ 5) + 32 = (252 ÷ 5) + 32 = 50.4 + 32 = 82.4 °F
