John wants to spend the saturday outside with his kids and their friends, but he can't decide what to do. he asks his kids and their buddies to see what they are interested in. 10 children want to go to the beach, 8 want to stay at the pool, and 15 want to go to the water-park. 4 of those children want to go to the pool and the water-park. 6 want to go to the water-park and the beach, and 2 want to go to the pool and the beach. 1 child wants to fit in all three. how many children does john plan to spend the day with?