This riddle has been around for some time on internet platforms such as Instagram, Facebook, and WhatsApp, and many people still argue over what the answer might be, as there are many possibilities. The correct answer is The professor has to add the rest of the digits, find the nearest number to the sum that is divisible by 9 and get the difference. So, John gave the number 9646 to the professor. The professor will add the numbers (9 + 6 + 4 + 6) to get 25. The nearest number to 25 that is divisible by 9 is 27. And the crossed out number is 27 – 25. This is a Maths trick that relies on the power of 9.