The initial sequence is 5 7 8
there is a missing number that goes between 5 7 and 8
there are 4 numbers , the first one is 5 and the last one is 8
the general formula is An = A1 + (r) (n-1)
but in this special case we find by observation that
theres ONLY ONE number between 5 and 8
is not 4 , also not 9 because they are outside the group
also is not 8 because is repeated
then we have only one option remaining that is 6.
6 is the answer searched.
Now
