Instructions: Click the "Count up" button to advance a counter and click the "Count down" button to decrement the counter. The current count shows up in the "number" textfield as an ordinary, base 10, decimal number. That same number is shown in the "hex" textfield as a base 16, hexadecimal number. It also shows up in the "binary" textfield as a base 2, binary number.
Decimal numbers require 10 symbols per digit - 0,1,2,3,4,5,6,7,8,9. Binary numbers require 2 symbols per digit so it is natural to use 0,1. But hexadecimal numbers require 16 symbols per digit. The 10 symbols of decimal numbers are naturally used to represent the lower valued digits. The symbols representing values 10,11,12,13,14, and 15 have been chosen to be A,B,C,D,E, and F, respectively.