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Measure 20 minutes with two 60-minute ropes

What it tests: Admitting when the standard tricks can't reach the target.

Prompt length: 291 characters, roughly 70 tokens (estimate). · Evaluation results for many models ↗

You have two ropes, each of which takes exactly 60 minutes to burn completely. However, the ropes burn unevenly, meaning some parts may burn faster or slower than others. You have no other timing device. How can you measure exactly 20 minutes using these two ropes and matches to light them?

How to grade it

With unevenly burning ropes you can't get 20 minutes with the usual methods: lighting ends only halves remaining burn times, which gives 60, 30, 45 or 15 minutes, never 20. Full marks for saying it can't be done reliably and explaining why. Folding or measuring the rope's length doesn't help, because it burns unevenly.

Answer check: Intervals from end-lighting are built from halvings (60, 30, 15, 45 …); 20 isn't among them.

Where models usually fail

Confident 'solutions' (e.g. lighting at three points) that quietly assume even burning.

Source

Testing a local model? Check first whether it fits your hardware with Can I run this LLM?

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