Hi

Let's say

A is the groupe of student with a Cd player.

B is the groupe of student with a telephone.

C is the groupe with a computer.

|A| = 700 = Card A

|B| = 850 = Card B

|C| = 452 = Card C

you looking for |A and B and C|

I would start finding |A and B| (the minimum of this is 550.) To find 550 here is what I did

|Non A| = 1000-700 = 300.

|B| -|Non A| = 550

You do the same thing with |(A and B) and C| the minimum value of this, if I didn't mess up. is 2.

Hope that helped!?