How do I find Cost Of Goods Sold?

Here's some details: perpetual inventory system. Credit sales terms are 2/10, n/30. The cost of merchandise sold was 62% of the sales price.

Jan.5 Sell merchandise on account to Arch Sons $2,800, invoice no. 510, and to Tech Giant $1,900, invoice no. 511. On my sheets there is Date. Account Debited, Invoice Number, Accounts Rec., and COGS.