Brain Teasers
The Rope Bridge
The torch-and-bridge puzzle as a story: speeds of 1, 2, 5 and 10, and a storm 17 minutes out.
How it works
Four friends reach a gorge at night with one torch. The bridge holds at most two people, whoever is on it must have the torch, a pair moves at the slower walker's pace, and the torch cannot be thrown back. Their crossing times are 1, 2, 5 and 10 minutes, and the storm hits in 17. The story asks for the minimum time to get all four across. You lock in a guess, then step through the answer and breakdown.
How scoring works
No grading - your guess is displayed back beside the answer. The story teasers share one lifetime unlock for free users.
The key insight
The instinctive plan - the fastest walker escorts everyone - costs 19 minutes because the 10 and the 5 are both paid in full. The winning move sends the two slowest across together so the 5 is absorbed inside the 10, at the price of an extra fast return trip: 1+2 across, 1 back, 5+10 across, 2 back, 1+2 across, for 17 exactly.
The full step-by-step solution lives at /brain-teasers/bridge-crossing/solution.
A worked example
Dee (1) and Cass (2) cross: 2. Dee returns the torch: 1. Amal (10) and Bo (5) cross together: 10. Cass returns: 2. Dee and Cass cross again: 2. Total 2 + 1 + 10 + 2 + 2 = 17, with the two fastest handling every return trip and the two slowest making one shared journey.
Common mistakes
• Answering 19 - the ferry-with-the-fastest plan feels efficient but pays both slow crossings separately.
• Pairing each slow walker with a fast one, which wastes the fast walker's speed entirely.
• Stopping at the first schedule that works instead of asking where the expensive minutes overlap.
Why interviews test this
Interviewers watch for the moment you question the greedy assumption: computing 19, noticing it misses 17, and hunting for overlapped cost is the optimisation instinct being tested. The written solution is at /brain-teasers/bridge-crossing/solution.