2/10 EOM means that the purchaser will receive a 2% discaount if he pays within 10 days after the end of the month.
It is supposed that if it bought on July 29, EOM is August 31,
Then, September 4 is within the terms to get the 2% cost and it paid Jones 400 - 2% of 400 = 400*0.98 = 392.