Need some guide in this question.

We did the usual way as below but my nephew n I couldn’t get the correct answer.

35-5 = 30 question

assume all correct

 30x 3=105

diff in marks= 90- 70

  =20

difference in scoring =3+2

= 5

questions wong 

=20/5

=4

30-4=26 questions correct

Appreciate anyone to point out the error. Thanks