Amazing 1089...

Step 1
Take two pieces of paper and hand one to a friend.
On yours, without letting them see, write the number 1089, then fold the paper to keep it hidden.
Step 2
Ask them to think of a 3-digit number but, before they write it down, ask them to put the numbers in order from greatest to smallest. Don't let them show what they've written.
Example: 543
Step 3
Below their number, ask them to write the same digits, but in reverse order, from smallest to greatest.
Example: 345
Step 4
Now, ask them to subtract the new lower number from the original one they wrote.
Example: 198
Step 5
Next, ask them to reverse the order of that number.
Example: 891
Step 6
Then, get them to add this latest number and the previous number together and show you the result.
Example: 891 + 198 = 1089
Step 7
Finally, you can reveal your own number, which (if they have calculated correctly) will be exactly what they have written...
1089
Amazing!

Maths Trick

STEP 1
Ask a friend to write down a number (any number with more than 3 digits will do, but to save time and effort you might suggest a maximum of 8 digits).
Example: 83 972 105
STEP 2
Ask them to add the digits.
Example: 8+3+9+7+2+1+0+5 = 35      
STEP 3
Ask them to subtract this number from the original one.
Example: 83 972 105 – 35 = 83 972 070
STEP 4
Ask them to select any digit from this new number and strike it out, without showing you.
Example: 83 972 070      
STEP 5
Ask them to add the remaining digits and write down the answer they get.
Example: 8+3+9+7+0+7+0 = 34
STEP 6
Ask them to tell you the number they get (34) and you will tell them which number they struck out.
SOLUTION
The way you do this is to subtract the number they give you from the next multiple of 9. The answer you get is the number they struck out.
Example: The next multiple of 9 here is 36 (9 x 4 =36)
36 – 34 = 2 and there you have your answer, easy isn’t it!
Note: If the number they give you after step 5 is a  multiple of 9, there are two possible answers  then you simply tell them that this time they crossed out either a 9 or a zero.