Find the missing next number: 4, 6, 9, 13, 18, ?

Explanation

1. 01

   Differences: $6 - 4 = 2$, $9 - 6 = 3$, $13 - 9 = 4$, $18 - 13 = 5$.

2. 02

   Each difference is one more than the last, so add 6: $18 + 6 = 24$.