Perpetual inventory using FIFO

Beginning inventory, purchases, and sales for Item Zeta$9$ are as follows:

Oct. $1$ Inventory

Oct. $7$ Sale

Oct. $15$ Purchase

Oct. $24$ Sale

$44$ units@$$18$

$34$ units

$42$ units@$$21$

$15$ units $31.$

a$.$ Cost of goods sold on October $24$

$x$

b$.$ Inventory on October $31$