Full volume of the rectangular tank = 55 × 40 × 36 = 79 200 cm³.

• Water in the tank = \(\frac{4}{5} \times 79\,200 = 63\,360\) cm³
• Volume poured into the cubical tank = 20 × 20 × 20 = 8000 cm³
• Water left = 63 360 − 8000 = 55 360 cm³
• 55 360 cm³ = 55 L 360 ml

Final Answer: 55 L 360 ml