20% of group young people says it's old = false
10% of old farts say their young = false
34% of total group (young + old) says they are old = While their is a 30% false in that group. So you take away 30% of the false of the actual old group and u calculate the amount of students by distracting the actual old group from 5000.
30% of 34% is 21%. 21% of 5000 = 1050 (actual old people) which leaves us 3950 students.