Hello.
Your question is about chemistry subject giving you the proper solution of this question  is difficult somewhere by writing here so I would suggest you to refer the link provided below and directly you can see the solution for your question.
https://learn.careers360.com/engineering/question-solve-it-when-benzoic-acid-dissolve-in-benzene-the-observed-molecular-mass-is/amp/#aoh=15901491389540&referrer=https%3A%2F%2Fwww.google.com&amp_tf=From%20%251%24s
Best of luck.