I am confused on the calculation for cost fo goods sold for the following problem:

E8-9 (Periodic versus Perpetual Entries) Fong Sai-Yuk Company sells one product. Presented below is information for January for Fong Sai-Yuk Company.

Jan. 1 Inventory 100 units at $5 each

4 Sale 80 units at $8 each

11 Purchase 150 units at $6 each

13 Sale 120 units at $8.75 each

20 Purchase 160 units at $7 each

27 Sale 100 units at $9 each

Fong Sai-Yuk uses the FIFO cost flow assumption. All purchases and sales are on account.

c) Assume Fong Sai-Yuk uses a perpetual system. Prepare all necessary journal entries.

The journal entry for sales on the 13th is:

A/R 1050

Sales 1050

Cost of Goods Sold (20 x $5) + (100 x $6) 700

Inventory 700

I am confused on to why we only take 20 units from the Jan 1 inventory and not 100 and why we take 100 units out of the Jan 11 purchase.