The double it and add 30 way is a quick, but rough mental calculation that's only OK in a limited rage of common weather temperatures. When the Met office was changing from Farenheight to Celcius they used to quote it for our quick conversion reference.
The other interesting thing about the quiz question is they gave a small clue to the solution in the question itself. The number 50 is one of the easiest to work out in your head, because you have to divide by 5 first, the answer ten is then easy to use to multiply the 9.
For these quiz types problems, I always look for such clues. If you need Excel to work it out, chances are you are on the wrong path.