what is the perpetual inventory

PROBLEM SET B Ming Company uses a perpetual inventory system. It entered into the following purchases and sales trans- actions for April. (For specific identification, the April 9 sale consisted of 8 units from beginning inven- tory and 27 units from the April 6 purchase; the April 30 sale consisted of 12 units from beginning inventory, 3 units from the April 6 purchase, and 10 units from the April 25 purchase.) Problem 6-1B Perpetual: Alternative cost flows