I have a daughter who I like doing math problems with. We often try to do like multiplication problems that are initially too difficult to do in our heads. We practice breaking the problem up into a progressively larger set of easier problems, and then simplifying and repeating the process as necessary.

It uses math.js to evaluate the expressions.

The key functionality is that on each new line, you get to edit the expression, as long as the value is ultimately unchanged. Once the expression string matches the value the problem is marked done.

It’s running at wiccatech.com/mathcheck and the source is at GitHub.