What are FIFO and LIFO?

Look at these transactions and determine what would be COGS: A merchandiser purchases the following inventory:

Jun 1: Purchase 1 book for $20 to resell to its customer

Jun 5: Purchase 1 book for $25 to resell to its customer

Jun 10: Purchase 1 book for $12 to resell to its customer

Jun 15: Sold 1 book to a student for $50.

What is the COGS for the book sold on Jun 15 under FIFO, LIFO COGS under FiFO .............................., Gross Profit under FiFo ..................................... COGS under LiFo .............................., Gross Profit under LiFo.......................................