Step 1 (2 minutes). Adventurers A and B cross to the far side together.
Step 2 (1 minute). Adventurer A runs back to the start with the torch. (Total time: 3 minutes)
Step 3 (10 minutes). Adventurers C and D cross to the far side together. (Total time: 13 minutes)
Step 4 (2 minutes). Adventurer B runs back to the start with the torch. (Total time: 15 minutes)
Step 5 (2 minutes). Adventurers A and B cross to the far side together one last time. (Total time: 17 minutes)