Rectangular tank measures 6 metres x 5 metres x 4 metres. How many litres of water can it hold?

The volume of a rectangular tank is given by the formula:

$$V = \text{length} \times \text{width} \times \text{height}$$

Given dimensions: length = $6\, \text{m}$, width = $5\, \text{m}$, height = $4\, \text{m}$.

$$V = 6\, \text{m} \times 5\, \text{m} \times 4\, \text{m}$$ $$V = 120\, \text{m}^3$$

We know that $1\, \text{m}^3 = 1000\, \text{litres}$.

$$V_{\text{litres}} = 120\, \text{m}^3 \times \frac{1000\, \text{litres}}{1\, \text{m}^3}$$ $$V_{\text{litres}} = 120000\, \text{litres}$$

The rectangular tank can hold $\boxed{\text{120000 litres}}$ of water.