What is the next number in the series 1 9 18 25 27 21?

The series appears to be following a pattern where each number is the result of a mathematical operation on the previous numbers. In this case, the pattern seems to be adding consecutive odd numbers starting from 1.

1 + 8 = 9 9 + 9 = 18 18 + 7 = 25 25 + 2 = 27 27 - 6 = 21

Following this pattern, the next number in the series would be 21 + 1 = 22.