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Puzzle | Torch and Bridge
Puzzle | Torch and Bridge
(taken from https://www.geeksforgeeks.org/aptitude/puzzle-18-torch-and-bridge/)
(taken from https://www.geeksforgeeks.org/aptitude/puzzle-18-torch-and-bridge/)
Last Updated: 21 Jul, 2025
There are 4 people (A, B, C, and D) who want to cross a bridge at night.
A, B, C, and D take 1, 2, 5, and 8 minutes, respectively, to cross the bridge.
There is only one torch with them, and the bridge cannot be crossed without the torch.
There cannot be more than two people on the bridge at any time, and when two people cross the bridge together, they must move at the slower person's pace.
Can they all cross the bridge in 15 minutes?
Check if you were right - full answer with solution below.
Solution: They must cross the bridge in the following way:
Step 1: A and B cross the bridge. A comes back. Time taken is 3 minutes. Now B is on the other side.
See the top image (step 1) to the right
Step 2: C and D cross the bridge. B comes back. Time taken: 8 + 2 = 10 minutes. Now C and D are on the other side.
See the middle image (step 2) to the right
Step 3: A and B cross the bridge. Time taken is 2 minutes. All are on the other side.
See the bottom image (step 3) to the right
Total time spent: 3 + 10 + 2 = 15 minutes.
To minimize the time:
The trick here is that only the people with the fastest speeds should come back (and only if there is a need to return, as here we need to bring back the torch). A comes back in step-1, and B comes back in step-2. And finally, reduce the number of trips back; for example, like C, D does not come back.
The River Crossing Problem (Farmer, Fox, Chicken, and Grain)
The River Crossing Problem (Farmer, Fox, Chicken, and Grain)
A quintessential logic puzzle, the River Crossing Problem is a staple in interview brain teasers, used to evaluate a candidate’s ability to plan, sequence, and solve problems within constraints. The scenario involves a farmer who must transport a fox, a chicken, and a sack of grain across a river in a boat that can only hold the farmer and one other item at a time. The key constraints are that the fox cannot be left alone with the chicken, and the chicken cannot be left alone with the grain.
The solution requires a series of non-obvious steps, including returning the item to its original shore. The key is to take the chicken across first, return alone, take either the fox or grain across, and then, crucially, bring the chicken back. This counterintuitive move is the linchpin to solving the puzzle and demonstrates a candidate's flexible thinking.
Why It's a Great Interview Question
Why It's a Great Interview Question
Interviewers use this puzzle to assess systematic thinking and planning rather than raw computational skill. It reveals how a candidate approaches a multi-step problem with interlocking dependencies.
Constraint Management: Can the candidate identify and remember all the rules? Do they test their proposed moves against these constraints before proceeding?
Systematic Approach: A strong candidate will not guess randomly; they will work through the problem sequentially, often verbalizing their steps and backtracking when a path fails.
Interviewer Insight: "We use this puzzle to see how candidates handle operational constraints. It’s not about finding the answer quickly; it’s about demonstrating a methodical, step-by-step process. Seeing them bring an item back shows they can think non-linearly to reach a goal."
This puzzle is a favorite in interviews for consulting and logistics roles because it mirrors real-world scenarios in which projects have strict dependencies and require careful, non-linear planning to avoid negative outcomes.
This video provides a step-by-step animated walkthrough of the solution, clearly illustrating the sequence of moves needed to safely move everyone and everything across the river.
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