My corrections are shown below:
He needed to give the correct answer to at least half of the questions and he had answered more than a half of the questions them. As the time was finishing, he decided to stop and to transfer his answers to the answer sheet.
After the exam, he was able to check the mark sheet in order to see if his answers were correct. His performance in Geography was a disaster. Two out of 10 answers were correct / right . His performance in History was not so different. Three out of 10 answers were correct. The answers for Biology, Mathematics, Chemistry, Physics, and English were fine. He gave correct answers to more than a half of the questions. However, it was not enough to compensate for his mistakes in Geography, and History. There was only one subject left to check: Portuguese. For his happiness, he gave correct answers to 18 out of the 20 questions provided, achieving the necessary number of correct answers he needed to pass in the exam.