What are some mathematical tricks?

Answer 1

Pick any number.

Multiply by 6.

Add 13.

Subtract 7.

Divide 3.

Divide 2.

Then subtract by the original number you chose.

Your result will always be 1.

Tell a friend to mentaly multiply their age by 2.

Tell them to add 14 then divide 2.

Ask your friend what number they currently have.

Now, you mentally subtract 7 from the number they tell you, then you'll know your friend's age.