Calculate the total mass of 7 lorries if each lorry has a mass of 3t 245kg. Express your answer in tonnes and kilograms.

We know that $1 \text{ tonne (t)} = 1000 \text{ kg}$.
The mass of each lorry is $3\text{t } 245\text{kg}$.

$$3\text{t } 245\text{kg} = (3 \times 1000)\text{ kg} + 245\text{ kg}$$ $$= 3000\text{ kg} + 245\text{ kg}$$ $$= 3245\text{ kg}$$

To find the total mass, multiply the mass of one lorry by the number of lorries.
Total mass = $7 \times 3245\text{ kg}$

$$= 22715\text{ kg}$$

$$22715\text{ kg} = 22000\text{ kg} + 715\text{ kg}$$

Since $1000\text{ kg} = 1\text{t}$, then $22000\text{ kg} = 22\text{t}$.
So, the total mass is $22\text{t } 715\text{kg}$.