(a) That's right. A statement made during the formation of a contract becomes a binding term of the contract if it can be inferred from the circumstances that the statement was intended by the parties to be legally binding. Statements made explicitly in the form of promises make this intention clear.
In other cases, where the statement is not in the form of a promise, the necessary intention can only be inferred from the circumstances in which the statement was made.
In this case, it is important to notice that the buyer explained to the seller that he needed paint with good coverage qualities. In reply to this, the seller made an unequivocal statement - he said that a 20 litre tub would cover 100 square metres. In these circumstances there can be little doubt that the parties intended this statement to become a term of their contract of sale.