1. **Problem:** Calculate the capacity of a tank that is 50 dm long, 3 dm wide, and 20 dm deep, when it is \(\frac{3}{5}\) full. 2. **Formula:** Volume of a rectangular tank is given by \(\text{Volume} = \text{length} \times \text{width} \times \text{depth}\). 3. **Calculate total volume:** \[ 50 \times 3 \times 20 = 3000 \text{ dm}^3 \] 4. **Convert volume to liters:** Since 1 dm³ = 1 liter, total capacity is 3000 liters. 5. **Calculate capacity when \(\frac{3}{5}\) full:** \[ \frac{3}{5} \times 3000 = 1800 \text{ liters} \] 6. **Answer:** The tank holds \(1800\) liters when it is \(\frac{3}{5}\) full.