If Jon starts the night with 20 dollars in his cash register and ends the night with 47 dollars in his cash register, then he must have earned 27 dollars during the night.
Since Jon sells comic books for 3 dollars each, we can divide the total amount of money he earned by the price of each comic book to find the number of comic books he sold:
27 dollars / 3 dollars per comic book = 9 comic books
Therefore, Jon sold 9 comic books during the night.