Question

An annoying question. (Reposted as original post was missing some information)

Answer 1

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 ?