The mass of the bottle plus the mass of the gasoline is 1,000 grams. And the mass of the bottle plus twice the mass of the gasoline is 1,600 grams (because the acid is twice as dense as the gasoline). So the difference, 600 grams, is the mass of the bottle’s volume of gasoline. The mass of the bottle full of gasoline is 1,000 grams, so the mass of the bottle alone is 400 grams. (This checks out: It tells us that the mass of the acid is 1,600 – 400 = 1,200 grams, which is twice the mass of the gasoline.)

From the March-April 1993 issue of Quantum (page 32, “Kaleidoscope”).