The question is:

On January 8, Quastrar, Inc. sent Hylavian Company a letter offering to sell $10,000 in restaurant supplies. On January 18, Hylavian mailed a letter to Quastrar accepting the offer. Quastrar received the acceptance letter on January 20. On January 17, Quastrar sent a letter revoking the offer. Hylavian received this letter on January 21.

I don't understand why there was a contract on 18. According to mailbox rule, the revocation is effective when the letter is mailed, so i think there was no offer because i was revoked on 17.