The corrected paragraph is:

There have been many studies which have shown the different pastimes of tertiary students. According to Timms (1991), participants listed watching television, dancing, and playing volleyball as their favourite activities. They spend the majority of their time during the week using the Internet and watch television for a significant portion of the weekend. The students provide a fascinating source of information for Timms, who has studied this topic for many years. He has interviewed no fewer than 200 students and claims that over the past 20 years everybody has answered his questions. “Each student spends about 15 hours of their time on Facebook and other forms of social media,” he said. “I don’t know if they realise how much this affects their marks.” Timms offers alternative pursuits that may be more beneficial to students, such as mentoring and reading.