There are 12 people in Mrs. Hilt's living room. 6 people are wearing socks and 4 people wearing shoes. 3 people are wearing both. How many people are bare feet?

3 people wearing both are also counted in the 6 with socks and 4 with shoes so you can't count them twice. What you do in stead is list : 
3 wearing both 
4 - 3 = 1 wearing just shoes 
6 - 3 = 3 wearing just socks 
total 7 so far so that leaves 5 others that must be barefoot.