Step 1 - Ask your friend to write the same 3 digit number on a paper and ask to keep it secret. For example, 111, 222, 333, 444, 555, 666, 777, 888, 999
Step 2 - Now tell them to add those three digits and keep it secret. For example - If your friend wrote 333 then 3 + 3 + 3 = 9
Step 3 - Now let's say the same 3 digit to divide it by its joint. for example - 333/9
Step 4 - Your answer will always be 37.