Optimizing 'Coin Pressure'

Don't know what I mean by coin pressure? Click here to read the previous post. Now, there are two possible things you might want to do with your coin pressure.

The first case is that you want to maximize your coin pressure. Why would you want to do this? Get rid of the change you are carrying of course. This is easily achieved by merely carrying a lot of change around - but most of us don't carry a coin purse, or anything that can hold substantial amounts of change. So what is it you can do to maximize coin pressure?

What you can do - is to always carry around a fixed set of coins and notes. Your meals would usually not exceed 10 dollars. (If they do, then it might be time you stop caring abount coin pressure and start caring about wtf you're eating.) This means that you should carry around enough change to pay anything from $5 to about$15 (just in case there was a want or need to splurge on drinks, dumplings, soup, or whatever nonsense was.)

Let's simplify the problem to just carrying $4.95 in coins - you can carry a$5 note and a $10 note to make sure you can cover the ranges extremes at$5 and $10. To be able to cover every possible change situation from$0.05 to $4.95, in Australian coins: • 1 x 2 dollars • 2 x 1 dollar • 1 x 50 cents • 1 x 20 cents • 2 x 10 cents • 1 x 5 cents This is obviously a very specific case, since there's not many 1 cent coins in circulation in Australia (if any) - so in order to maximize coin pressure, you only need to carry 8 coins in total. Very manageable for most wallets (unless you don't have a coin pouch or holder). If you live in a country where there are no$2 coins or notes, and you do still have 1 cent coins, then to cover $0.01 to$4.99 you would need to carry:

• 4 x 1 dollar
• 1 x 50 cents
• 1 x 20 cents
• 2 x 10 cents
• 1 x 5 cents
• 4 x 1 cents

13 coins - which could pose a problem. But if you live in Malaysia - you have RM1 notes! (and 1 sen coins are gonna get phased out eventually anyway, but 5% taxes at food places make it annoying.) And if you have $1 notes, you then only have to carry 9 coins - which is again manageable. Of course, this guide is only good if you happen to have a huge stack of coins at home and want to get rid of them - if you keep reloading your wallet from your stash like this, you'll see to drop in size quickly enough. Obviously, this guide only works if we assume coins are in some decreasing order of size (barring the Australian dollar and two dollar coins). It's entirely possible that this is not always true (for example, the US nickel, 5c, is a lot larger than the US dime, 10c, and there's that 25c coin that'll just annoy your math if you're used to 20c coins.) - just adjust accordingly. The second case is that you want to minimize your coin pressure (or in other words, create a change vacuum.) Why in the world would you want to take change? There are various reasons - number 1 - you need the change, since the notes you are carrying are way too big. This usually happens if you're walking about with 50 dollar or 100 dollar notes - which are almost always too big to spend on anything small (sub-$10).

It's pretty simple to achieve - carry around very big notes, or if you feel like turning your friends into portable ATMs (that are sometimes difficult to collect from, like malfunctioning ATMs ;) ) - carry around a credit card, preferably one from the same country. Don't want to get caught up in messy exchange rate risk now, do we?

(Note: Look - no photo. What did I tell you about me not maintaining any trend?)

Random Thought 11th May 2008

Look properly at the one who hasn't been seen entering but has always been there. Look again at the one who has been seen inside but hasn't been seen entering.

Look once more at the one who has been inside but doesn't have a map.

Look once more - and you will find the one who knows everything.

Praying to Resin Statues

In 2006, I made an impulse decision to buy two resin statues of Magic: the Gathering characters. One was of Eight-and-a-Half-Tails and another was of the Kodama of the North Tree. While I'm not gonna post photos of them, since they're no longer out on show like they used to be in my Arrow apartment, and they were pretty stupid purchases since nobody I knew appreciated them. Anyone who knows me also knows that I'm pretty bad at controlling my spending. I buy loads of random stuff on a whim, and it's usually worked for me.

I tend to think a lot when buying stuff, and this is one of the two really impulse purchases I've ever made. The other one is a story for another day. So, like always, I bring back some bulky box into my small Arrow apartment, stuff them into the storage (my apartment was a corner, so it had a small storage space). Anyway, I took them both out to examine. I found one of them lightly damaged, nothing I couldn't fix - so I put up the other one with a view to alternate every once in a while.

Needless to say, having a fox on somewhat of a stage/pedestal that looks like some kind of extravagant deity in addition to my already slightly strange behaviour - led to my housemate asking me whether or not I pray to the thing. And he wasn't kidding. *sweat*

After moving to my new place, I didn't display either since I didn't have the table space for them. (Arrow had these low drawers that I couldn't really do much with, so I just put the resin statue on one of them.)

Well, so here, there are in storage and not doing anything at all! Cool eh? Well - at least no one will think I pray to a nine-tailed fox deity and a monstrous looking forest spirit - right?

Extending the Traffic Light Problem

Of course, I looked at a simplified version of the first part of the problem. There are always more traffic lights in real life. Here's the big picture (and the real problem):

Now that you know the big picture - does my original argument still hold? To see whether it does, let us consider what happens when we choose to cross at the first traffic light if it is green.

If we do, our problem is simplified to this:

Using our original assumptions of independent Bernoulli trials, you can see simply that you have a 87.5% chance of being able to cross at one of these set of the moment you come to one. This are significantly better odds than you had before (50%) - but it is still unable to beat the result from skipping that first green traffic light (100%). (Yes, crossing at that north T junction vertically DOES NOT require you to wait for a traffic light.)

Minimize the number of traffic lights you have to cross to minimize travel time.

Why is this important? It will help you understand some decisions you can choose to make during my journey to university when I get to that - and the path I chose to minimize travel time, while minimizing disutility while travelling to university. Next up: why you might decide against lower travel time.

The Law of Coin Pressure

I remember have this strange theory that is usually true. I might have even mentioned it on this blog before maybe even blogged about, but this is a post entirely devoted to this funny subject.

In a contracted form, it simply says this: The more coins or small change you have, the more likely you are to give them out - and the less likely you are to receive them. I call it "coin pressure" as a reference to air pressure. Air flows from high pressure areas to low pressure areas - and so do coins. This is typically the case when you go out with friends.

Imagine the case where you go out with friends to eat. Being students, going Dutch is standard practice. There is no arguing over who pays the bill. Everyone pays the bill. Now, some places do cater to students and allow you to pay separately for items on the bill. This means that there is conversation going: "You owe this much from that, and you're supposed to pay for that, and I'm supposed to pay for this."

Usually you relegate this calculation to the accounting major students by just using the excuse that usually goes: "Let's give this to the accounting majors to sort out." In real life, there's probably a better mental mathematician at the table than the accountants (in fact I think in our case it was a management major student) - but hey, anything to not do the work. :P

After it gets sorted out, then everybody gets the numbers. Sometimes the numbers can get pretty bad change wise, for example, $8.90 per person. That means to pay exactly, you need one$5 note, one $2 coin, one$1 coin, one 50c coin, and two 20c coin - and that's already the simplest possible. 6 pieces of change to pay off your bill.

Of course, if you have the change, you just pay your part. If you don't, you take on more and more change - in general, the bigger the currency note you have to pay with, the more change you will have to carry when you leave. It follows that the opposite holds true, the more small change you have, the less small change you will have to carry when you leave. (Hence, the origin of the law.)

There is the largest possible denomination of currency - a credit card. If you are paying the bill on behalf of the table by credit card, then you effectively collect all the payment by everyone else - since you had the lowest "coin pressure".

Of course, there is such a thing as too much coin pressure. If you carry around 200 5c coins just so that you will always have enough coin pressure to pay off any possible bill within \$10 - nobody will want to have to split a bill with you. :P

Moral of the story?

Carry a good variety of coins and small notes with you.

Try to never leave any coins at home unless it's in your piggy bank. Coins at home have this bad habit of accumulating to ridiculous numbers since you leave more and more and home - and leaving coins at home reduces your coin pressure, hence increasing the probability of you receiving even more coins.