Forty students were asked about the communication tools they most often use such as e-mails, cellular phone, and social networking sites. The survey result are as follows: 20 students use e-mails, 27 students use cellular phones, 30 students use social networking sites, 12 students emails and cullular phones, 15 students use emails and social networking sites, 18 student use cellular phones and social networking sites and 8 students use all three tools.
1. How many students use emails only?
2. Cellular phones only?
3. Social networking sites only?
4. Cellular phones but not emails?
5. Neither of the three tools?