Question

What is the volume of a substance whose density is 11.3g/ml and mass is 25.3g?

Answer 1

Density = mass / volume
so
volume = mass / density

Answer 2

BluFoot gives a good answer. This is good example of how to work things out by paying attention to the units: if the units of density are g/ml then dividing a quantity in g by the density will give an answer in ml. Likewise if the volume had been given then the mass could have been calculated by multiplying the volume by the density - the units of volume would have cancelled out to give an answer in grams.