# Some fun (easy!) maths for (a little) rep!Watch

#1
Some fun questions I thought you might like:

1. If I take a square and split it into 4 similar smaller squares, and remove one of them, how can I split the remaining shape into 4 equal shapes? Can you do the same with an equilateral triangle? (You can show me your answers in Paint.)

2. A gamber plays at a casino (which had unlimited funds). He starts with Â£x. Each turn, he flips a coin. If it lands heads up, he wins Â£1, but if it lands tails up, he loses Â£1. With a fair coin, the game would be expected to last forever. However, the casino (naturally!) is using a loaded coin, that has a probability 'p' of landing heading (p<0.5). How long can we expect the game to last for now, before the gambler runs out of money (in terms of p and x)?

3. There are 100 doors, arranged in a circle, all of which start locked. A man walks around, unlocking every door, and then walks around again, locking every other door. He then goes around again, locking or unlocking every third door. The man continues doing this, until he has locked/unlocked every 100th door (i.e. the last). Which doors are now unlocked?

Each question earns (a little) rep, but more importantly, you get the satisfaction of doing maths!
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12 years ago
#2
3) i answered this a while ago in a 'questions you might get asked at a cambridge interview' thread. now give me rep.

2) E(x) = p - (1-p) = 2p-1

n is number of goes, we want nE(x) = x = n(2p-1)
n = x/(2p-1)

1) which triangle is removed in the second case? and edge or the middle one?
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12 years ago
#3
3. All the square numbers because of the repitition of factors, 1, 4, 9, 16, 25, 36, 49, 64, 81 and 100
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#4
Chewwy:

Well, you seem to be close enough on it... I'll give you your rep in due course!
I did work out something slightly different for 2, but if you can give me some proper working, I'll give you the rep anyway

And for 1) - the triangle removed has to be one on the edge. It probably isn't possible to do it for the middle one...

ssmoose: I've given you rep anyway!
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12 years ago
#5
ssmoose: I've given you rep anyway!
Cheers, I prefer the set up with the coins rather than the doors though.
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12 years ago
#6
Q1 is attached.
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12 years ago
#7
Ah that's why I couldn't do question one and it seemed ridiculous. I figured once you'd cut the square in 4 the original lines would remain.
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