Final answer:
The ratio in Foakes v Beer was that part payment of a debt is not sufficient consideration for a promise to forgive the remainder of the debt; consequently, Mrs. Beer could claim the interest from Dr. Foakes despite their agreement.
Step-by-step explanation:
The case of Foakes v Beer is a landmark decision in English contract law. The ratio decendi, or the legal reason for the decision, was that part payment of a debt is not sufficient consideration for a promise to forgive the remainder of the debt. In this case, Dr. Foakes owed Mrs. Beer a sum of money, and they agreed that he would pay the debt off in installments without interest. Later, Mrs. Beer sued for the interest, and the court decided in her favor, stating that Dr. Foakes's agreement to pay the original debt, which he was already legally bound to pay, did not constitute new consideration for Mrs. Beer's promise to forgo the interest.