I thought this was kinda cool

[tazber]

Posted: 7/11/11 at 1:50pm

Take the last two digits of your birth year and add them to the age you will be on 12/31/11.

For example:

Birth year 1983
Age 28

Add 83 plus 28.

What do you get? Post your answers. Only the final sum, we don't need to know when you were born or how old you are.

(this will only work during this year, 2011)

Updated On: 7/11/11 at 01:50 PM

[ahhrealmonsters]

Posted: 7/11/11 at 2:21pm

It's 111 for everyone, right?

[madbrian]

Posted: 7/11/11 at 2:44pm

As long as we're talking about 'cool' math, I'll try one. It's a visual, so I may hae trouble conveying it, but I'll try. It's a way to do multiplication by 9 on your fingers. Hold your hands up in front of you, fingers splayed. To see the answer for 9x4, lower the 4th finger from the left (your left index finger). The result is three fingers, space, six fingers, or 36. Works for 9x1 through 9x9.

[orangeskittles]

Posted: 7/11/11 at 4:42pm

madbrian, why didn't you post that when I was in 4th grade? The 9 times table was a killer!

[jasonf]

Posted: 7/11/11 at 5:51pm

A math trick that takes a little practice but is cool once you get it:
To square any two digit number in your head
1)Round the number to the nearest 10 (let's say 23 - so 20)

2)Find the "opposite" of the rounding by adding or subtracting the same amount you rounded by: 23 went down 3 to 20, so up 3 would be 26.

3)We now have two new numbers that are fairly easy to multiply in your head: 20x26. Easy way to do this:
20 x (20+6)
20x20=400;
20x6=120;
add those together, 520.

4)Now you still have that 3 that you rounded by to deal with. Square that and add it to your total: 529. 23x23 is 529.

It works with every two digit number. Some are easier to do than others (squaring anything with a 5 is super easy), but with practice you can get to do these fairly fast. There's a trick to starting to answer while you do the calculations in your head that makes it look really impressive. I used to be able to do these quickly, but I'm out of practice.

Another example, just to clarify:
74 squared. We round up and down by 4.
70x78.
70x70=4900; 70x8=560. 4900+560=5460. Add the 16 from what you rounded, you get 5476.

[singtopher]

Posted: 7/11/11 at 6:13pm

An additional note about Taz's post, it only works for those born in 1999 or earlier. If anyone here is was born 2000 or later (god help us) you have to add 100.