Thanks! I'm not making money off the practice problems though. In fact, the practice app (https://mentalmathpro.com/mental-math-practice) is free.

I think of it more as a short book (it's priced like one) but has the added benefit that you can practice each lesson as many times as you need.

That's why the strategies are gated, they're the most important part. But you do have a point that the first three are the least interesting... Maybe I should offer the first lesson in each category first, or something like that.