Think of a number

Between 1 and 10

Multiply it by 2.

Add 5.

Multiply by 50.

If you've already had your birthday this year
Add 1755.

If you haven't had your birthday this year
Add 1754.

Now subtract the 4 digit year you were born.

You should be left with a 3 digit number.

Cover up the last two digits with your finger.

The first digit is the number you thought of.

And you can never lie about your age.

Because it is under your finger.