Students in your class can use 4f Checker to find errors in their own solution. It gives them immediate feedback while they are working on the problem. This can be minor calculation errors, formulas and theories used in the wrong way, or simply writing the wrong numbers. They get help with locating the problem, and will be able to correct some of the errors themselves, while everything is still fresh in their mind. They can send their solutions to you and you can then help them with the more substantial problems, the issues they have not understood. This will remove the obstacles to learning, and your students are ready to learn new things. You can also use 4f Checker to find errors in your students' solutions, or to check your own model solutions.