Which number fills the gap in 6, 11, ?, 27, 38, 51?

Explanation

1. 01

   Use the complete end of the series: $38 - 27 = 11$, $51 - 38 = 13$. The differences rise by 2.

2. 02

   Working back, the differences are 5, 7, 9, 11, 13, so the missing term is $11 + 7 = 18$.

3. 03

   Check: $18 + 9 = 27$.