An empty 60 litre petrol tank has a mass of 10kg. Its mass when full of fuel of relative density 0.72 is

The volume of the tank is $V = 60 \text{ litres}$.
Since $1 \text{ litre} = 0.001 \text{ m}^3$:

$$V = 60 \times 0.001 \text{ m}^3 = 0.06 \text{ m}^3$$

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:

$$\rho_{\text{fuel}} = \text{RD}_{\text{fuel}} \times \rho_{\text{water}}$$ $$\rho_{\text{fuel}} = 0.72 \times 1000 \text{ kg/m}^3$$ $$\rho_{\text{fuel}} = 720 \text{ kg/m}^3$$

The mass of the fuel ($m_{\text{fuel}}$) is its density multiplied by its volume:

$$m_{\text{fuel}} = \rho_{\text{fuel}} \times V$$ $$m_{\text{fuel}} = 720 \text{ kg/m}^3 \times 0.06 \text{ m}^3$$ $$m_{\text{fuel}} = 43.2 \text{ kg}$$

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 $\text{53.2 kg}$.