A125 ml flask and stopper has a mass of 55.263 g. a 25.00 ml sample of gasoline is pipetted into the flask. the final mass is 73.385g. what is the density of the gasoline?

Density is is defined as the amount of substance per a defined volume of the substance.  That being said to find density you would divide the mass of the substance by the volume the substance takes up.  In this question, the mass of the gasoline would be 18.122g since you subtracted the final mass of the flask (73.385g) by the initial mass of the flask (55.263g) to find the amount of gasoline added.  The volume the gasoline takes up is 25 mL as stated in the question.  That means that the density of the gasoline is 0.72488g/mL since 18.122g÷25mL=0.72488g/mL.  This answer makes sense since gasoline should have a slightly lower density than water which has a density of 1g/mL.

I hope this helps.  Let me know if anything is unclear.