Explanation

To find the next number in the series 11, 14, 18, 21, let's analyze the pattern of differences:

• $14 - 11 = 3$
  
• $18 - 14 = 4$
  
• $21 - 18 = 3$
  

The differences alternate between +3 and +4.
Following this pattern:

• After $21$, add 4: $$21+4=25