Brain-hacking calculations make Road to Olympia contestants kneel down to calculate

This is a question in the Road to Olympia program with a very high level that makes the contestant have to send a lot of money to calculate.

In the Finish Line contest of contestant Minh Hoang, a difficult calculation appeared, causing all 4 climbers to give up. The question had the following content: "A is a natural number with 2020 digits. Knowing that A is divisible by 9, B is the sum of the digits of A, C is the sum of the digits of B, D is the sum of the digits of C. Find D?"

Right after the MC asked the question, contestant Minh Hoang knelt down in the middle of the stage to make it easier to calculate. The male student from Hanoi gave the correct answer as 9, however the answer was incorrect. The remaining contestants did not give an answer to this calculation.

The brain-hacking calculation made the contestant of Road to Olympia kneel down to calculate.

If you find the answer to this problem, scroll down to the comments to post it and see how many people think the same as you.

Source: https://vtcnews.vn/phep-tinh-hack-nao-khien-thi-sinh-duong-len-dinh-olympia-phai-quy-goi-tinh-toan-ar931104.html