Application of DE

Hello, do you have any solved difficult sample problems about Newton's law of cooling for practice?

The topic is chemical reaction rate.

Two substances A and B are combined to form a product C. The formation of product is proportional to the time the reactants are combined. The final product is composed of two parts of B for every part of A. If initially A is 30 kg and B is 20 kg, and 5 kg of the product is formed after 30 mins., find the function of product formed at any given time.