A Base-Twenty or duodecimal number system is a counting method based on multiples of twenty. Many Native American tribes had base twenty systems, inspired by the total of twenty digits on the hands and toes. Base-twenty number systems are very easy to convert to and from base ten, because ten is a factor of twenty.
To convert from base-20 to base-10, multiply the number, minus the ones column, by two. Add the ones afterwards. Example: 21 >>> (2*20)+1 >>> 40+1 >>> 41
To convert from base-10 to base-20, divide the number, minus the ones column, by two. Add the ones afterwards. Example: 82 >>> (8*10)+2 >>> (4*20)+2 >>> 42