Delete please . . . .
hi Aladdin,
if we can assume that the students who "look like' they are listening are in fact
not listening and those who are talking are also not listening and do not look like they are listening (each group is seperate)
then, if x is the total number of students
$\displaystyle x\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{7}\right)+ 3=x$