Inventory Methods under the Periodic Inventory System

Robin Inc.$\text{'}$s inventory records show the following data for purchases and sales of units.

The actual ending units remaining at May $31$ were purchased on the following dates:

Required

Assuming that Robin maintains a periodic inventory system with a monthly physical count, compute ending inventory on May $31$ under $(1)$ specific identification, $(2)$ average cost, $(3)$ FIFO, and $(4)$ LIFO.

$(1)$ Specific identification

$-$Note: Do not use negative signs with any of your answers.

$(2)$ average cost $($periodic inventory system$)$

$-$Note: Use the result EXACTLY as displayed above in the calculations below.

$-$Note: Do not use negative signs with any of your answers.

$(3)$ FIFO $($periodic inventory system$).$

$-$Note: Do not use negative signs with any of your answers.

$(3)$ FIFO $($periodic inventory system$).$

Note: Do not use negative signs with any of your answers.

$(4)$ LIFO $($periodic inventory system$).$

Note: Do not use negative signs with any of your answers.

I need to know why lifo and fifo's ending inventory are calculated the way they are.