The original rock problem was, you have 12 rocks, of which 11 are of the same weight, and one is either lighter or heavier than all the rest. You have a scale and you can use it three times. How will you find which is the odd rock out?

It drove us crazy for a few days and lots of the colored rocks in the rock bowl in the couch place went missing.

So, the next level up is, you have 39 rocks, 38 are of the same weight, and you don't know if the last rock is heavier or lighter. Find this rock using the scale four times.

posted by Tae-Yeoun Keum   [11:04 AM]

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