Cbse Board Maths Exam Paper Query

Thank you so much @Thyagarajan and @Gauri03 for your explanation. I understand the mistake now. My daughter is upset only because she thinks what she did is right and that its unfair if she looses marks for a right answer. Her hurt will reduce if she knows her marks came down because of a genuine mistake. Will show these replies to her after all her exams are over. Yes disappointments are part of life. We learn something with every failure. It's just that as a mother, I know how much effort she put despite her troubled health and so many other problems through out the year, not to mention 2 board exams this year( since last year the board exams had to be cancelled due to covid, the govt introduced 2 terms so that if there is a surge in covid cases there will be at least one term marks to take in to account). She got a very good percentage in the first term and got full in maths. My child has come a long way after all the bullying, physical and mental issues. So I was looking forward to some victory to cheer her up. May be there are better victories waiting. Thanks a lot for all the replies here and the time taken by you all in giving different approaches to this query of mine.