Assuming that every 10 minutes there will be a new block on the blockchain and you will be rewarded 50 bitcoins in the first 4 years after you solving the math problem. Your reward will be 25 bitcoins in the second 4-year period if you build a new block, and the reward will be reduced by half every 4 years. The starting year of Bitcoin mining is 2009. William would like to know the number of Bitcoin still remained starting from the year 2021.