Jiangsu, Jiangsu, Jiangsu Education Edition, fifth grade, second volume, mathematics exercise book, page 59, answer to the first question.
On March 5th, 580KG was actually used, and 1000KG was purchased, leaving 420KG. The actual surplus of 720KG minus the surplus of 420kg in March = 300kg (surplus in February), and 420-300= 120(KG) plus 120KG.