Don't ever go in for these quizzes, as they are a big, big con. They are really just a random number and they keep on letting people guess until they've raised enough money from the phone ins. If the call costs less than a pound, the company running the quiz do not have to justify how they got the answer, but you can bet it will not be a simple addition sum.
For example, NINETEEN - FIVE also includes the Roman Numerals I, I, IV and V, but this is just one of the many weasellings. Never ever phone in for one of these late night 'do the sums' quizzes.