That's the one I got, and I'd argue, along with you, that that is the best. Of course any of these sequence problems can have multitudinous answers, and it is a judgement call as to which is the best.
In the spirit of The Hitchhikers Guide to the Universe, someone has figured out how 42, the answer to life. the universe, and everything is an answer: The nth term of this sequence is given by the formula
f(n) = -(31/24)n^4+(55/4)n^3-(1145/24)n^2+(285/4)n-33
I don't know how I missed that one! Someone obviously has too much time on their hands.
The solution of 78 implies that if the sequence was extended to the left, the next term would be 1/2.
Can you think of a sequence where 1 is added as the first (leftmost) term?
1 3 8 18 38 ?
This one is not a mathematical formula.