I just got a possible solution from a friend,
7 12 27 47 \_
7 12 27 47 \_ calculate difference
5 15 20 \_ calculate difference
10 5 \_ -> a possible sequence can be taken here, => 0 -5 -10...
so the following 5 15 20 \_ becomes 5 15 20 20
that way we get the 67 in the question as the answer.
this seems logical, but is it a correct way to solve such a problem ?