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## Math for fun and glory

### Course: Math for fun and glory>Unit 2

Lesson 1: Brain teasers

The perfect logicians are at it again. Created by Sal Khan.

## Want to join the conversation?

• Could you solve this brain teaser with more than 3 numbers, and could those numbers be anything other than the ones that were suggested in this video???
• Yes but it would be much harder with even 1 more person.
(1 vote)
• I doesn't seem to work out for me if I choose numbers other than 20 and 30 (or another 2/3 ratio. For example with 25 and 45. I can be 20 or 65, so the last guy will be 5 or 65 if I am 20, or 40 or 90 if i am 65. All those options are legal, so I will still be undecided. So I am thinking it only works when b-a = a-(b-a) or when a=2/3b. Am I right here?
• A big part of the problem is that you are able to figure out what your number is knowing the other two can't figure out their number. This won't be the case for any 3 numbers.
• is the answer 50 because it feels like the unknown is the sum of the other numbers
• It does not have to be that way but yes.
(1 vote)
• Hey, here's a brain teaser/lateral thinking puzzle of my own: There are 2 rocks. Both of them are EXACTLY 1 billion years old, yet one is older than the other. How?

In a few days, I will be posting the answer, if none of you guys can figure it out.
• well, you'd be... sort of right. i mean, if they were both exactly 1 billion years old, maybe one of them was still aging, one second older. because there's no such thing as the present. how? think about it. if you were to stop time, you'd be frozen too. but put it to normal, everything i in the past. when you talk, you say it, and it's in the past.
(1 vote)
• Here's a brain teaser-
3 perfect logicians are taking a nap under a tree. While they are napping, a small boy smears their noses with red berries. When they wake up, they begin to laugh, thinking the other two were laughing at each other. But then, one of them stops laughing, realizing that his nose is red too. How did he come to this conclusion?

/!\ WARNING!
The answer is not too obvious.
• Let’s call the philosopher’s A, B and C. A reasoned that B was confident his nose wasn’t red. If B saw A’s nose wasn’t red, he would be surprised that C was laughing, because C would have nothing to laugh at. But B wasn’t surprised, therefore, A correctly reasoned his nose was smeared.
XD
• at is minus 10 equal to negative 10? This brain teaser was awesome!
• Yes, subtracting ten is also equal to negative ten, if you've done algebra before.
(1 vote)
• A bomb just exploded in the middle of a city. 32 people survived and over 20,000 people died. The survivers needed supplies to live. One brought water, one brought food, one brought clothes, one brought a pet, and one brought a phone. The rest of the people were thinking on what to do. One person said they could go to texas and ask for help. The other option would be to call for help from the guy who brought the phone. Imagine you were in this situation, what would you do given the supplies you have.
• One would assume that this was not New York City. If I drove to Texas from New York City, it would be a long time.