...A high number like 30 requires a sequence that can vary like 1, 9, 2, 8, 4 and 6. The goal still being that the sum of the sequence adds up to 30. Because there is a limit to how many numbers can be summed, the limit of the target numbers is 40.
As the game progresses and your score increases, the game becomes progressively harder by increasing the range of target numbers and reducing the available time.