Duodecimal number system works by counting up 0,1,2,3,4,5,6,7,8,9,A,B, then moving to the next decimal place, examples below:

123 in duodecimal would be A3

84 in duodecimal would be 70

8645 in duodecimal would be 5005, and so on...

Try to relearn how to count, add, subtract, multiply, and divide, and remember how hard it it for kids everyday to learn the basics of math!