Sure it is.
How much mass did you put in the cup ? (12.8g + 11.4g) = 24.2 g
What is the triple-beam-balance reading ? (20g + 0g + 4.5g) = 24.5 g
Pretty close to equal, I'd say.
The only problem I have with this question is this: First it says that you 'dissolve' one substance in another one, and then it says you measure the 'mixture'. If you dissolved it, then you have a solution, not a mixture.
