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Tri-Valley Skeptics Meetup Message Board › Riddles for Robby (and whomever else likes riddles)

Riddles for Robby (and whomever else likes riddles)

Tom C.
Tombolian
Antioch, CA
Post #: 75
1. You have two ropes each of which takes exactly 60 seconds to burn. You also have as many matches as you need. You need to measure 45 seconds. How can you accomplish this?

2. You have 9 identical-looking coins, one of which is actually heavier than the others. You also have a balance scale.
What is the least number of times (uses of the scale) required to ferret-out the heavier coin?

3. You have 12 identical-looking coins, one of which is either heavier or lighter than the rest (you don't know). You also have a balance scale. You can only use the scale 3 times. How do you determine both a)which coin is the odd one and b)is it heavier or lighter?

Good luck!
Robby
robby.tjahjono
Pleasanton, CA
Post #: 69
1. You have two ropes each of which takes exactly 60 seconds to burn. You also have as many matches as you need. You need to measure 45 seconds. How can you accomplish this?

OK Tom, I think I have the answer. Beginning at the same time, light the 1st rope at both ends, and light the 2nd rope at only one end. That 1st rope will take 30 seconds to burn up. At the moment the 1st rope is burned up, the 2nd rope will still have 30 seconds to go. Immediately light the 2nd rope at the other end. It should completely burn up 15 seconds later. 30 seconds from 1st rope + 15 seconds from 2nd rope = 45 seconds total!

2. You have 9 identical-looking coins, one of which is actually heavier than the others. You also have a balance scale.
What is the least number of times (uses of the scale) required to ferret-out the heavier coin?

I've answered this last Friday, so I'll leave it unanswered here for the others to take a crack at.

3. You have 12 identical-looking coins, one of which is either heavier or lighter than the rest (you don't know). You also have a balance scale. You can only use the scale 3 times. How do you determine both a)which coin is the odd one and b)is it heavier or lighter?

I'll think about this one some other time. Hopefully I'll be able to solve it before 2015 =).
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