The sequence 240, 280, 320 is an AP with common difference:

1. Subtract one term from the next. 2. 280 minus 240 is 40, and 320 minus 280 is 40 too. 3. So the common difference is 40. 4. The answer is 40. _Source: syllabus Mathematics, Ch 8 'Predicting What Comes Next: Exploring Sequences and Progressions'_