Explanation:

- Pipe A fills the tank in 30 hours, which means it fills 1/30 of the tank in an hour.

- Pipe B fills the tank in 36 hours, filling 1/36 of the tank in an hour.

- Pipe C fills the tank in 28 hours, filling 1/28 of the tank in an hour.

- All three pipes work together initially.

- A is closed 5 hours before the tank is full.

- C is closed 8 hours before the tank is full.

- Let's assume the tank is filled in "t" hours.

Working period:

- Pipe A works for (t - 5) hours.

- Pipe C works for (t - 8) hours.

- Pipe B works for the entire "t" hours.

Equation:

$$

(t - 5) \left(\frac{1}{30}\right) + t \left(\frac{1}{36}\right) + (t - 8) \left(\frac{1}{28}\right) = 1

$$

Solving the equation gives t = 15 hours.

Option 4 (15) is correct.

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