Marissa Had 500 Shares in XYZ Corp That She Was

Marissa had 500 shares in XYZ Corp that she was willing to sell at a price of $10 each.
She wrote a letter to Mark that said, "I will sell 500 shares in XYZ Corp to you at a price of $10 each.You can accept any time within the next month.This is a firm offer." Marissa signed and sealed that document.When he received that letter two days later, Mark decided to accept Marissa's offer.He wrote a letter to that effect, but then forgot to mail it.Two weeks later, the value of the shares increased to $20 each.Marissa therefore no longer wanted to sell them at the original price.She telephoned Mark and told him that her offer was revoked.She was entitled to do so.