How do you beat the bridge crossing game

Here are the steps:

1. Send across: 1, 3
2. Send back: 3
3. Send across: 8, 12
4. Send back: 1
5. Send across: 1, 6
6. Send back: 1
7. Send across: 1, 3

Each number represents the character that will take that many minutes to cross the bridge.