If 2 girls are filling 1 cup up equally with fluids? And 1 drinks it and then fills the cup up with a solid and the other girl proceeds to eat it and then refill the cup with and exact matter... will they both equally weigh the same as before they started?
A somewhat oddly worded question.
I take the pronoun 'they' refers to the girls and not the cups, or is it just one cup?
Provided the girls are not losing mass in any way then any difference in their mass will depend on their intake. As liquids can have different densities (milk has a different density to water) then there will be a different gain in mass from two different liquids. Similarly as solids have a different density to liquids then a cup of a liquid will have a different mass to the same volume of a solid. Thus there will be a different gain in mass.