This is a bad question. Not enough information is given. We do not know the number of moles of original carbon dioxide in the mixture. As such, even if we can calculate the additional number of moles of carbon dioxide, we cannot find the total number of moles of carbon dioxide and hence, we cannot find the mole fraction of carbon dioxide. If this is a standard 'A' level question, you should consult with your teacher to inform the Cambridge Exam authorities to see whether there is a flaw in the question.