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InterSystems·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Got a classic brain teaser during a software engineering interview at InterSystems. Not the kind of thing I expected to spend 10 minutes sweating over, but here we are.

Questions Asked (1)

Q1

You have two ropes that each burn completely in 60 minutes, but they burn unevenly. Using only these ropes and a lighter, how would you measure exactly 45 minutes?

Algorithms & Data StructuresAdaptability & Ambiguity
Author's notes

The 60-minute part is trivial, just burn one rope.

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AI HintsAI Generated

Suggested Approach

Start by clarifying that the ropes burn unevenly, so you cannot rely on length or burn rate. The key is to light both ends of one rope and one end of the other simultaneously; when the first rope burns out (30 minutes), light the other end of the second rope, which will then burn out 15 minutes later, totaling 45 minutes.

Pro tip: After presenting the solution, mention that this puzzle tests the ability to reason about time and state under uncertainty, which is directly applicable to designing algorithms that handle non-uniform data or unreliable inputs.

1. Understand the constraints

Acknowledge that each rope burns in 60 minutes but unevenly, so you cannot measure time by length or assume a constant burn rate.

2. Identify the key insight

Realize that lighting a rope at both ends makes it burn in half the time (30 minutes), regardless of uneven burning.

3. Plan the sequence

Light both ends of Rope A and one end of Rope B at the same time. When Rope A burns out, 30 minutes have passed.

4. Execute the second step

At that moment, light the other end of Rope B. Rope B will now burn from both ends and be completely consumed in 15 more minutes.

5. Confirm the total time

The total time from start to when Rope B burns out is 30 + 15 = 45 minutes.

Key Points to Mention

  • Lighting a rope at both ends halves its burn time to 30 minutes, regardless of uneven burning.
  • The first rope (both ends lit) acts as a 30-minute timer.
  • When the first rope burns out, 30 minutes have elapsed, and the second rope has 30 minutes of burn time remaining if burned from one end.
  • Lighting the other end of the second rope at that moment makes it burn from both ends, so it will be consumed in 15 minutes.
  • The total time is 30 + 15 = 45 minutes.
  • This approach works because the burn rate, though uneven, is consistent for each rope, so burning from both ends always takes half the total burn time.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.