Okay, so here's the deal. Here's a table.

Number - Answer
[10 - 19] - [9]
[20 - 29] - [18]
[30 - 39] - [27]
[40 - 49] - [36]
[50 - 59] - [45]
[60 - 69] - [54]
[70 - 79] - [63]
[80 - 89] - [72]
[90 - 99] - [81]

Choose a number. So let's say, you chose 37 (O_O) anyways, you chose 37. What you have to do is:
Step 1:
A = ?
The number you chose, which is 37 in this case.
A = 37


Step 2:
A1 + A2 = B
Where A1 is the first number of the number you chose, and A2 is the second. B is the second variable.
3 + 7 = (10)

Step 3:
A - B = Answer.

So, that would be:
37 - 10 = 27. The answer! Cool huh?

[What I think:]
All the number from 10-19, or 20-29 if added together, are always equal to a multiple of 9. Why? Because A1 and A2 is always below 9.

I'm not sure.

Can someone please explain how that worked? I don't know why, but it ... works. o_o

Well, since the base 10 stays the same for each answer... if you increase the "ones" column by one, it will increase the amount you subtract from by 1 as well (when you do the A1+A2 it will increase by one, making B one greater) but since the A is one higher, and B is one higher, they cancel each other out, leaving the same answer until the "tens" column changes.

Basically its just making you increase the left side and right side of the minus sign both by the same value, thus making it X+Z - Y+Z making it always equal X - Y, with the Z's being the increment within the span of numbers evaluating to the same answer.

I have no clue if my explanation made it clear or not haha, kinda rushed it

I used to love doing these little mathematical parlor tricks and head games when I was in grade school...the teachers would try to "amaze" us with math...it was kinda pathetic, since the teachers who were doing them were just reading them out of a book, and had no idea how they worked.

Haha I was the kid the teachers hated because I could figure them out and break them usually, or at least explain them better then the teacher It was always a blast making the teachers angry by doing things they couldn't react to mwhaha

Another cool thing I like about the number 9 is this:

1/9 = 0.1111111111...

Now we can say that 1/9 + 1/9 = 2 * 1/9 (nobody disputes this). So to solve for 2/9, we can multiply the above by 2:

1/9 * 2 = 0.1111.. * 2 = 0.22222...

So likewise, we can solve for 3/9, 4/9, etc.

The tricky thing is when we get up to 9/9. This is obviously 1. 9 divided by 9 is 1. 9 * 1 = 9. However, we can also express it as 9 * 1/9. Which is:

9 * 1/9 = 9 * 0.111111... = 0.9999999...

So then 1 = 0.9999999999...?!?!?!?!

INTENSE!

An infinite series of 9's is the same as 1! What implications does this have for mathematics?