Some of you geeks probably have a slide-rule in your head — there I go dating myself again (and we all know that dating yourself isn’t sexy), I should have said “math processor” or “calculator”.  Anyway, I am personally much better than average at math, but I still can’t do some of the bigger problems in my head.

Via Mercola, I found this list of 10 Easy Arithmetic Tricks to save you the trouble of getting out a calculator for solving some math problems.

The “11 Times Trick” is one I hadn’t thought of before, but it makes perfect sense.  To multiply any two-digit number by 11, you add the digits together and put the result between them, adding any excess digit to the first digit.  It makes sense because multiplying it out long-hand you get (for any two digit number mn):

m   n
X   1    1
———-
m   n

m   n
————
m(m+n)n

Some of the tricks are a bit obvious, like the multiply/divide by 5 and how to compute a 15% tip (a lot of places are starting to expect 20% now anyway).  But all the tricks are good exercises for understanding how numbers work.

Here are a few more tricks that I learned somewhere along the way for determining if a decimal integer is a multiple of:

1. Duh.  Of course it is.
2. Is the last digit even?  Another easy one.
3. Add up the digits in the number.  If the result is a multiple of 3, then so is the number.  If the sum of the digits is too big for you to know whether it’s a multiple of 3 or not, recurse.  Add up its digits and see if that’s a multiple of 3.
4. Take the number formed from the last two digits.  If that’s a multiple of 4, so is the number.  Why?  Because 100 is a multiple of 4.
5. Is the last digit a 0 or a 5?  Too easy.
6. Is the number a multiple of 2 and 3?
7. Double the last digit and subtract it from the number formed by the remaining digits.  If that result is a multiple of 7, so is the original number.  If you can’t tell, recurse.  For instance, take 357.  Double 7 to get 14, subtract that from 35 and you get 21.  Since 21 is a multiple of 7, so is 357.
8. Take the number formed from the last three digits.  If that’s a multiple of 8, so is the number — because 1000 is a multiple of 8.
9. Add up the digits in the number.  If the result is a multiple of 9, then so is the number.  Again, you can recurse if you’re not sure.  It’s no accident that this rule for 9 is the same as the rule for 3.
10. Does it end with a 0?  Now we’re back in elementary school.
11. Add up all the odd digits to get one number, then add up all the even digits to get a second number.  If the difference between them is a multiple of 11 (zero included), then so is the number.  If you think about it, this is really the “11 Times Trick” reversed.  Let’s take 26719 as an example.  2 + 7 + 9 = 18, 6 + 1 = 7, 18 – 7 = 11, so 26719 is a multiple of 11.
12. Is the number a multiple of 3 and 4?

Looking at this list, the relationship between 3, 6, and 9 is obvious — as is the relationship between 4 and 8.  The unique characteristics of 7 and 11 are intriguing, don’t you think?

What other arithmetic tricks do you know?

# Vista Promo: No one does bloatware like Microsoft

The Folks at Fridaypage just released a funny Vista promo video that I’m sure Microsoft wouldn’t approve of. While I don’t necessarily agree with most of what the video says, it still made me laugh a couple of times. Enjoy!

# Turn your IM client into a domain checking tool

I’ve previously posted about Imified but I have recently noticed a new tool of theirs that you can add to your IM client. It quickly becomes very useful if you are a heavy IM user and you also do a lot of domain checking.

As you can see from the menu on the left, you can ping a domain, perform a traceroute, get Alexa domain information, perform a WHOIS check and also carry out an “enum” search (I’m not sure what this is – can anyone explain?).

The results are fast and it is extremely convenient.   A worthy addition to any IM client.

# Burn calories in your hot tub with the AquaFit 19DT

Progress never stops. The proof? This all-in-one gym/hot tub hybrid that allows you to exercise and relax in the comfort of your own home.

The Aquafit 190T is a gigantic tub divided in two parts. The largest one is a mini-gym in which you can do cardiovascular activities such as rowing, running or stationary swimming. The second side, which can be set at a different temperature and features adjustable hydrotherapy jets, will allow you to relax after your calorie-burning exercise session.

As far as I’m concerned, the only setback is the price: \$40,000 + installation fees. A bit too pricey for a modest system administrator, like myself!

# So Long, HD DVD

A few years ago when I first heard of you, I thought you had the coolest name. It was like love at first sight. You gave me great hope for the long-term, I believed in everything you said, and I invested in our relationship. But today, I heard the terrible news. You literally broke my heart. Today, is the end of our relationship. Today, is the end of HD DVD.

Toshiba is expected to finally show the white flag in its war against Sony for the next generation DVD format.

# Experiment of the week: Why we sweat

If you pump iron regularly, or have been shoveling the damned snow out of your driveway for the past few months, you’ve probably wondered why people sweat in the first place. In the following video, mad scientist Robert Krampf shows us the mechanics behind this natural phenomenon.

# Integrating Skype into Pidgin

After a long time obsessing over Google Talk, I am now slowly turning my attention to Pidgin. I was inspired to take a closer look at Pidgin after an excellent tutorial on Lifehacker and so now I am experimenting with my Google Talk contacts on the Pidgin platform to see how it goes.

But I was disheartened to see that out of all the IM networks that Pidgin supports, the one it didn’t support by default was Skype. So in the interests of trying to be more organized in my life, I went hunting to see if there was any kind of a plug-in based on the Skype API which allowed Pidgin to support Skype contacts.

And there is! The Skype API Plugin allows you to add your Skype contacts to your Pidgin application. But there are some drawbacks :

• You can’t make phone calls through Pidgin. So this would only be suitable for Skype accounts which you use for instant messaging only.
• For the moment, you need to also have the official Skype application on (which kind of defeats the purpose of integrating it into Pidgin in the first place). But the developer promises to fix this in future releases.
• The plug-in imports EVERYTHING from your Skype list, including regular phone number contacts. This one actually forced me to uninstall the plug-in in the end as I have about 150 regular phone number contacts in my Skype app which were then imported into Pidgin. So this plug-in would be best for people who don’t use Skype for regular phone calls.

But if you only use Skype for instant messaging and you want to merge your contacts with your Pidgin contacts, then this is an excellent plug-in to install (it even shows the Skype contact’s icon). There’s even a Linux version.

Granted, it’s a pain in the rear to also need to have the Skype app open but I am hoping that in the future, new releases will take care of these minor irritants. In the meantime, you can hide the Skype app icon from the Windows system tray and pretend it’s not there.

If you’re a Skype and Pidgin fan, this is one plug-in to keep an eye on.

# Total Lunar Eclipse on Wednesday

Mark your calendars folks! Next Wednesday, starting at around 8:43PM EST, a total lunar eclipse will occur over the Americas. The next one is not due until December 2010, so if the sky is clear, get your derrière up from your computer chair and head outside to observe this rare phenomenon.

A lunar eclipse happens when the Moon passes through the shadow of Earth. You might expect the Moon to grow even more ashen than usual, but in fact it transforms into an orb of vivid red.

Why red? That is the color of Earth’s shadow.

[Via NASA]

# The ID theft break-in

In this segment of The Real Hustle, Jess shows why you should never, ever dump important documents in the garbage / recycling bin without having fed them to the paper shredder first.

# Home-made laser harp looks and sounds awesome

Created by US inventor Stephen Hobly, The laser harp is played by breaking various laser beams with your hands to create a unique, techno-like melody that is sure to fascinate your audience. Unfortunately, it’s not available for purchase yet, so if you want one, you’ll have to build it yourself.