There are four large groups of people, each with 1000 members. any two of these groups have 100 members in common. any three of these groups have 10 members in common. and there is 1 person in all four groups. all together, how many people are in these groups?
put equation (2) to equation (1)
-2y+1 + 2y=9
-2y + 2y=9-1