Explanation:

Let's solve the problem step by step:

- Pipes A, B, and C fill the tank in 10, 15, and 30 hours, so:

- A's rate = 1/10 per hour

- B's rate = 1/15 per hour

- C's rate = 1/30 per hour

- Combined rate of A, B, and C:

- 1/10 + 1/15 + 1/30 = (3 + 2 + 1)/30 = 6/30 = 1/5 of the tank per hour

- In 3 hours, together they fill:

- 3 × (1/5) = 3/5 of the tank

- Then D is opened, and it empties the tank in 30 hours now.

- D empties (3/5) tank in 30 hours.

- So, D's rate = (3/5)/30 = 1/50 per hour

- But D alone takes x hours to empty full tank.

- So, D's original rate = 1/x per hour

- Equating:

- 1/x = 1/50 ? x = 50

Options:

- Option 1: 40 (incorrect)

- Option 2: 50 (correct)

- Option 3: 60 (incorrect)

- Option 4: 45 (incorrect)

- Correct option is Option 2: 50