1.This is a case involving an individual named Sheldon. Sheldon had a legal dispute with his former assistant, Yu Yan. On March 1, Sheldon entered into a contract with Yu Yan for her to write and draft a textbook for his graduate class on "contract law". Yu Yan was to write the textbook by October 14 and, for her effort, she would be paid $50,000. Both parties agreed. On July 1, Sheldon became nervous about Yu Yan completing the textbook on time and went to see her. Yu Yan stated that she was extraordinarily busy, but if Sheldon really wanted to be sure that the textbook was finished on time, then he would have to pay her an additional $5,000. The most important thing for Sheldon was getting the textbook written on time and so he agreed. Sheldon breathed a sigh of relief and waited for Yu Yan to finalize the textbook. She completed the textbook on October 13 and is now demanding the $55,000 from Sheldon. However, Sheldon has refused to pay.

Ignoring the issue of discharge, is there valid consideration for the two contracts between Sheldon and Yu Yan?

Explain with full reference to the applicable law for the two contracts.