Instead of a base 10 (decimal) system like we use today, the Maya used a base number of 20 (vigesimal) in their mathematics. They also used a system of bars and dots as “shorthand” for counting. A dot stood for one and a bar stood for five.

 

Because the base of the number system was 20, larger numbers were written down in powers of 20 instead of 10 like we do today. For example, in our system, 32 is 3*10+2. In the Maya system, this would be 1*20+12 because they used 20 as the base.  It was very easy to add and subtract using this number system. They did not use fractions.  Adding was just a matter of tallying up dots and bars.  Maya merchants often used cocoa beans laid out on the ground to do these calculations.

 

(Adapted from source)