$5 + 10 - 2 = 13$ 
David and Billy are on the bus together. They sit in the same column. There are $5$ people in front of David, Billy is in the middle. There are $10$ people behind Billy, and David is in the middle. How many people in this column? (adapted from2004 Math Kangaroo Problem, Level 3 - 4, Question \#19) 

1) $$14$$ 
2) $$13$$ 
3) $$12$$ 
4) $$11$$ 
5) $$10$$ 