Actual paper link:
http://advances.sciencemag.org/content/5/2/eaav0961they can only increment or decrement a number by 1 ("honeybees were able to use color as a symbolic representation of the addition and subtraction signs and learned, during 100 appetitive-aversive trials, to thus add or subtract one element from different samples. ")
The paper is fairly readable even outside the area of expertise. It's straightforward behavioral work.
Amusingly they used sugar as the positive reward and quinine as the negative reward. I quite like the taste of quinine...