Explanation:
In this scenario, Newton owes Rs. 18,000 to Johnson. However, Johnson agrees to accept Rs. 10,000 as full settlement of the account. The agreement between Newton and Johnson is valid.

Valid Agreement:
A valid agreement is one that meets all the essential elements of a contract. The essential elements of a valid contract include:
1. Offer and Acceptance: There must be a clear offer made by one party and acceptance of that offer by the other party.
2. Intention to Create Legal Relations: The parties must intend to enter into a legally binding agreement.
3. Consideration: There must be something of value exchanged between the parties. Consideration is usually in the form of money, goods, or services.
4. Capacity to Contract: Both parties must have the legal capacity to enter into a contract. This means they must be of sound mind and not under any legal disability.
5. Free Consent: The consent of both parties must be free and not obtained through coercion, undue influence, fraud, or misrepresentation.
6. Lawful Object: The purpose of the agreement must be lawful and not against public policy.

In this case, all these essential elements are present. Newton owes Rs. 18,000 to Johnson, which constitutes an offer. Johnson's acceptance of Rs. 10,000 as full settlement indicates agreement and acceptance. There is also consideration involved as Newton is providing money to settle the debt. Both parties have the capacity to contract, and there is no indication of any coercion or fraud. Additionally, the agreement has a lawful object, which is to settle the debt owed by Newton.

Therefore, the agreement between Newton and Johnson is valid and enforceable. Johnson is legally bound to accept Rs. 10,000 as full settlement of the account, and Newton's obligation to pay Rs. 18,000 is discharged by paying Rs. 10,000.