Each bag of popcorn cost $2.50. I know this, because if Betty had $40 in her pocket and left with $10, then she spent $30 on popcorn. 30 divided by 12 equals 2.5. So, each bag of popcorn cost $2.50
If she started with $40 and ended up with $10, that means she must've used $30. Since the popcorn bags were all identical and she bought 12, you can divide 30 by 12 which gives you 2.5.