- A & B cross first (2 min)
- A leaves B on the far side in the dark and returns (1 min; total time: 3 min)
- C & D cross with the lantern (10 minutes for the pair, total time: 13 min)
- B returns with the lantern (2 more minutes, 15 total)
- Finally, A & B cross (2 more minutes; 17 total).

Of course, it took them 15 minutes to come up with this idea, by which time the zombies would have arrived. My brother-in-law suggested throwing the interviewer at the zombies and making a mad dash across the bridge...