Two Maths Games

Ask a friend to write down a number with more than 3 digits.
Example: 12345

Ask them to add the digits.
Example: 1+2+3+4+5=15

Ask them to subtract this number from the original one.
Example: 12345–15=12330

Ask them to select any digit from this new number and strike it out,
without showing you. Example: 1330

Ask them to add the remaining digits and write down the answer they get.
Example: 1+3+3+0 =7

Ask them to tell you the number they get (7)
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. If the final number they give you is a  multiple of 9, there are two possible answers: 0 or 9.
Example: The next multiple of 9 here is 9, 9–7 = 2 and there you have your answer.

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.

Ask them to think of a 3-digit number that cannot be the same backwards.Then ask them to put the numbers in order from greatest to smallest. Don't let them show what they've written.
Example: 931

Below their number, ask them to write the same digits, but in reverse order, from smallest to greatest.
Example: 139

Now, ask them to subtract the new lower number from the original one they wrote.
Example: 931-139=792

Next, ask them to reverse the order of that number.
Example: 297

Then, get them to add this latest number and the previous number together and show you the result.
Example: 297+ 792 = 1089

Finally, you can reveal your own number, which will be exactly what they have written.