Working alone, pipe A fills a tank in 6 hours and pipe B in 9 hours. Working together, they fill the tank in:

1. Identify what is being asked: this is a **Rates And Work** question. 2. Apply the standard approach for this question type to evaluate each option. 3. The correct answer is **C. 3.6 hours** — it satisfies the conditions of the stem. _Source: GMAC Official 12 Sample Questions (mba.com)_