To calculate the mass of the tank when full of fuel, we need to find the mass of the fuel and add it to the mass of the empty tank. Step 1: Convert the volume of the tank to cubic meters. The volume of the tank is $V = 60 \text{ litres}$. Since $1 \text{ litre} = 0.001 \text{ m}^3$: $$V = 60 \times 0.001 \text{ m}^3 = 0.06 \text{ m}^3$$ Step 2: Calculate the density of the fuel. The relative density of the fuel is $0.72$. The density of water ($\rho_{\text{water}}$) is $1000 \text{ kg/m}^3$. Relative density is defined as the ratio of the density of a substance to the density of water: $$\text{RD}_{\text{fuel}} = \frac{\rho_{\text{fuel}}}{\rho_{\text{water}}}$$ So, the density of the fuel ($\rho_{\text{fuel}}$) is: $$\rho_{\text{fuel}} = \text{RD}_{\text{fuel}} \times \rho_{\text{water}}$$ $$\rho_{\text{fuel}} = 0.72 \times 1000 \text{ kg/m}^3$$ $$\rho_{\text{fuel}} = 720 \text{ kg/m}^3$$ Step 3: Calculate the mass of the fuel. The mass of the fuel ($m_{\text{fuel}}$) is its density multiplied by its volume: $$m_{\text{fuel}} = \rho_{\text{fuel}} \times V$$ $$m_{\text{fuel}} = 720 \text{ kg/m}^3 \times 0.06 \text{ m}^3$$ $$m_{\text{fuel}} = 43.2 \text{ kg}$$ Step 4: Calculate the total mass of the tank when full. The mass of the empty tank ($m_{\text{empty}}$) is $10 \text{ kg}$. The mass of the tank when full ($m_{\text{full}}$) is the sum of the mass of the empty tank and the mass of the fuel: $$m_{\text{full}} = m_{\text{empty}} + m_{\text{fuel}}$$ $$m_{\text{full}} = 10 \text{ kg} + 43.2 \text{ kg}$$ $$m_{\text{full}} = 53.2 \text{ kg}$$ The final answer is $\boxed{\text{53.2 kg}}$.