Explanation:

- Let's call the slower pipe's rate of filling the tank "x" tanks per minute.

- The faster pipe's rate is "2x" because it fills the tank twice as fast.

- Together, their rate is "x + 2x = 3x" tanks per minute.

- They can fill the tank together in 15 minutes, so \( 3x \times 15 = 1 \) tank.

- Solving for x gives \( x = \frac{1}{45} \) tanks per minute.

- This means the slower pipe fills the tank alone in 45 minutes.

- Correct Answer: Option 2 - 45 minutes