40 gallons of liquid b weighs the same as 30 gallons of liquid a.
A substance's density is defined as its mass per unit volume (more specifically, the volumetric mass density; also known as specific mass). Although the Latin letter D may also be used, the sign most frequently used for density is (the lower case Greek letter rho). Density is mathematically defined as mass divided by volume.
where m is the mass, V is the volume and D is the density, which is sometimes roughly characterized as weight per unit volume, while this is incorrect scientifically; this quantity is more precisely known as specific weight.
For liquid a,
8= mass/30
For liquid b,
6=mass/volume
For the two masses to be equal,
240=6*volume
volume=240/6
volume=40
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