expected marks= $\sum\limits marks(Q_{i})*probablity(marks(Q_{i}))$
probabilty($marks(Q_{i})$)=(no of student that answered the question $Q_{i} $ right)/ total no of student
probability($marks(Q_{1})$)=21/44
probability($marks(Q_{2})$)=15/44
probability($marks(Q_{3})$)=11/44
probability($marks(Q_{4})$)=23/44
probability($marks(Q_{5})$)=31/44
hence,
expected marks=2*(21/44)+3(15/44)+(11/44)+2(23/44)+5(31/44)=6.795