Question

What volume of alcohol with the density 8.4*100 will have the same mass as 4.2m of petrol whose density is 7.2*100

Answer 1

The mass of 4.2 m³ of petrol:

$$m=\rho_p V_p=720·4.2=3024\text{ kg}.$$

The volume of alcohol for the same mass of alcohol:

$$V_a=m/\rho_a=3024/840=3.6\text{ m}^3.$$