At the Gooddog Obedience School, dogs can learn to do three tricks: sit, stay, and roll over. Of the dogs at the school: 50 dogs can sit & 17 dogs can sit and stay

29 dogs can stay & 12 dogs can stay and roll over

34 dogs can roll over & 18 dogs can sit and roll over

9 dogs can do all three & 9 dogs can do none.

How many dogs are in the school?