数学トレーニング is a free educational app that is perfect for middle school students to practice and improve their math and calculation skills. This app covers various fields of math taught in middle school, such as equations, functions, geometry, probability, and data analysis. It also includes review problems on decimals and fractions.

The app offers a range of problem levels to cater to different grades in middle school, as well as review levels for 4th, 5th, and 6th grade math. Students can challenge themselves with math problems at their appropriate level and track their progress.

One notable feature of 数学トレーニング is the ability to write intermediate calculations on the problem screen using your finger. If you need more space for writing, you can use the green "Notepad and Pen" button.

This app, "数学トレーニング (Middle School Math Calculation Study App for 1st, 2nd, and 3rd Grade)", provides all its problem sets for free. However, it displays advertisements as it is supported by an advertising network. If you encounter any mistakes or issues, you can contact the developer through the "Contact" section within the app.