Gulf Corp. made purchases of a particular product in the current year as follows :

Ending inventory consists of $160$ units, specifically identified as follows :

$100$ units from beginning inventory

$40$ units from the February $6$ purchase

$20$ units from the July $25$ purchase

Assuming a periodic system, determine the costs to be assigned to cost of goods sold and ending inventory using the following systems :

a$.$ FIFO

b$.$ LIFO

c$.$ Weighted Average

d$.$ Specific Identification.