## Sequence 1, 3, 7, 12

Which is the next number in the following sequence:

**1, 3, 7, 12, 18, 26, 35, 45, 56, ?**

**SOLUTION**

This is the so called Hofstadter Figure-Figure Sequence.

The sequence of the differences between the consecutive numbers in the original sequence is 2, 4, 5, 6, 8, 9, 10, 11… These are exactly the natural numbers missing from the original sequence. Therefore, the next number should be 56 + 13 = 69.